Structure and hydrogen bonding of 2-aminopyridine . (H2O)n(n = 1, 2) studied by infrared ion depletion spectroscopy

Ronghu Wu, Petr Nachtigally and Bernhard Brutschy*

Institut fur Physikalische und Theoretische Chemie, Johann Wolfgang Goethe-UniversitatFrankfurt, Marie-Curie-Str. 11, D-60439, Frankfurt am Main, Germany.E-mail: Brutschy@chemie.uni-frankfurt.de; Fax: þ49-69-79829560

Received 24th October 2003, Accepted 3rd December 2003F|rst published as an AdvanceArticle on theweb 6th January 2004

The electronic and vibrational spectra of the clusters of 2-aminopyridine (2AP) with one and two watermolecules have been measured by using resonant two-photon ionization (R2PI) and IR predissociationspectroscopy (IR/R2PI). Under microhydration the aromatic nitrogen forms a very strong hydrogen bondwith water. The H-bonded OH stretching vibrations for 2AP�H2O and 2AP�(H2O)2 exhibit a red shift of 254and 413 cm�1, respectively, relative to the symmetric stretching vibration of free water. In addition water formsan H-bond with the amino group, resulting in a cyclic structure for both clusters. However, due to a reducedgeometrical strain, the 2AP�(H2O)2 cluster exhibits considerably stronger H-bonds than the 2AP�H2O cluster.The results of calculations performed both at the density functional and perturbation level of theory are alsoreported. Theoretical results are in good agreement with the experimental data. In addition, they bring furtherinsight into the structure and energetics of the microhydrated molecule.

1 Introduction

Heterocyclic molecules containing nitrogen atom are funda-mental in many biological systems. Their interaction with asolvent, especially water, very often plays an important rolein their conformational and dynamical behaviour. Thus 2-ami-nopyridine (2AP) may be used as a model molecule relevantfor understanding biomolecules containing purines and pyri-midines. Isolated 2AP has been extensively studied by usinga variety of different techniques. Kydd and Mills measuredits microwave spectrum and found that the –NH2 group isout of plane in the ground state.1 High resolution photoelec-tron spectroscopy was applied to measure the ionizationenergy and vibrational features of the cation.2 High resolution,rotationally resolved optical spectra of 2AP have also beenmeasured.3 The rotational constants deduced for the groundand first electronically excited state were qualitatively similarto those of aniline, showing that its geometries are comparablewith that of aniline. Resonant two-photon ionization spectro-scopy (R2PI) and mass analysed threshold ionization spectro-scopy (MATI) have been used to study the properties in theexcited and cationic state. A very accurate adiabatic ionizationenergy was determined and some low frequency vibrationalmodes in both the excited and cationic state were observed.4

But hitherto, cluster studies on microhydrated 2AP have onlybeen reported by Hager and Wallace.5 As water may form anH-bond both with the nitrogen atom in the ring and with theamino group, one of the goals of this study was to determinethe structure of 2AP with one and two water molecules bymeasuring the H-bond sensitive vibrations in the NH andOH region. Moreover, by comparing the measured vibrationalspectra with quantum chemical calculations, additional prop-erties such as binding energies, bond lengths and higher energyisomers etc. could be deduced.

The combination of R2PI with time of flight mass spectro-metry is a well established method for investigating clustersof specific composition. However, it is difficult or even impos-sible to study hydrogen bonds with R2PI, because the stretch-ing vibrations of NH and OH groups involved in H-bonds aregenerally not excited in an electronic transition starting from avibrationally cold cluster due to very small Franck–Condonfactors. In order to overcome this problem, IR/R2PI iondepletion spectroscopy, combining cluster selective R2PI massspectrometry with vibrational predissociation spectroscopy,has been developed. It was first used to study the intramolecu-lar vibrations of benzene and its dimer by Page et al.6 LaterRiehn et al.7 proposed it as a general method for characterizinga microsolvation environment and applied it to determine thestructure of the precursor of a quantitative intra-cluster ion-molecule reaction. This IR/UV double resonance techniqueis meanwhile widely applied to study hydrogen bondinginteractions in microsolvated molecules.8–13

In this work, the R2PI spectra of 2AP�H2O and 2AP�(H2O)2have been measured. IR/R2PI ion depletion spectra of theclusters have been recorded in the NH and OH stretchingregion (3200–3800 cm�1). We have also performed quantumchemical calculations for obtaining the structures, bindingenergies and vibrational frequencies of the clusters in theground state. The results from the calculations agree well withthe experimentally observed vibrational spectra.

2 Experiment and calculation

The experimental setup for determining R2PI and IR/R2PIspectra has been described in more detail elsewhere.14,15 Var-ious solute�(solvent)n clusters, abbreviated as 1:n–clusters, areproduced in a supersonic expansion through a pulsed nozzle,operated at 10 Hz. The vapour of 2AP at about 40 �C, ismixed with the He seed gas, containing less than 1 vol%water vapour. The distribution of the clusters can be optimisedby controlling the relative content of water via a needle valve.The total stagnation pressure is about 3 bar. Typical operating

y Permanent address: Heyrovsky Institute of Physical Chemistry ofthe Academy of Sciences of the Czech Republic and Center forComplex Molecular Systems and Biomolecules, Dolejskova 3, 18223Praha 8, Czech Republic.

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R E S E A R C H P A P E R

T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 5 – 5 2 1 515

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pressures are 5� 10�6 mbar in the expansion chamber and2� 10�7 mbar in the detection chamber. Behind the skimmer,the cluster beam is ionized by one-colour R2PI and mass ana-lysed in a conventional linear time of flight mass spectrometer(TOF-MS). Due to the resonant step in the two-photon ioniza-tion, a R2PI ion-yield spectrum reflects the UV absorptionspectrum of the neutral precursor. The monomer and clustersare ionized by using the frequency-doubled output of an opti-cal parametric oscillator (OPO) working in the UV (Conti-nuum Sunlite). The amplified ion signals are collected andanalysed by a transient recorder (LeCroy 9310) interfaced toa personal computer. In order to increase the S/N the spectraare averaged over 150 laser pulses. 2-aminopyridine (2AP,99þ%) was purchased from Aldrich and used without furtherpurification.The IR spectra may be recorded by IR/UV double reso-

nance spectroscopy utilising a continuously tunable IR-OPO.The IR laser pulse precedes the UV laser pulse by about 100ns. The focused UV and IR lasers are counter-propagatingto each other and intersect the skimmed molecular beam at aright angle. If a cluster absorbs an IR photon in the 3000cm�1 region it rapidly dissociates. If the population is thenprobed with the UV laser, a dip in the corresponding ion signalis observed. The IR depletion spectrum of an individual clusteris recorded by scanning the IR wavelength while the UV wave-length is fixed to a transition, specific to the cluster understudy. This spectrum represents the vibrational transitions ofthe neutral cluster in the ground state.The IR laser light is generated by a home-built, injection-

seeded OPO using LiNbO3 crystals both for the OPO andthe difference-frequency mixing stage. Its wavelength may betuned in the range from 2.5–4.0 mm at a bandwidth of 0.2cm�1. The IR laser intensity has a reduction in the range of3460–3510 cm�1 because of the absorption from OH� insidethe crystal.16 The typical energy is about 5 mJ pulse�1. TheIR-depletion spectra were normalized with respect to the IRlaser power. The laser wavelength is regularly calibrated by awavemeter (Atos LM007). The error limits of the energy is�3 cm�1.Ab initio calculations have been carried out with the Gaus-

sian 03 program.17 The geometries of several minima on thepotential energy surface of 1:1 and 1:2 clusters were optimizedat the density functional theory level, employing the B3LYPexchange-correlation functional18,19 and the 6-311G(d,p) basisset. The harmonic frequencies calculated at the B3LYP/6-311G(d,p) level were scaled by a factor 0.98. The zero-point-vibrational energies (ZPVE) were calculated at the B3LYP/6-311G(d,p) without any scaling. The interaction energiesbetween 2AP and water molecule(s) were calculated at theB3LYP/6-311G(d,p) and MP2/cc-pVTZ levels,20,21 and werein both cases corrected for the basis set superposition error(BSSE) using the counterpoise correction method.22

3 Results and discussion

3.1 Ab initio calculation

Four cluster structures of 2AP�H2O and three of 2AP�(H2O)2 ,obtained at the B3LYP/6-311G(d,p) level of theory, aredepicted in Fig. 1. Some relevant structural parameters aresummarized in Table 1. The binding energies calculated atboth the B3LYP/6-311G(d,p) and MP2/cc-pVTZ levelstogether with zero point vibrational energies (ZPVE) andbasis set superposition error (BSSE) corrections are listed inTable 2.Two hydrogen bonds are formed between water and 2AP

molecules in the most stable isomer of the 2AP�H2O cluster.This isomer (denoted 1a in Fig. 1) forms a six-membered cyclicstructure. Here water is the donor in its H-bond to the aro-matic nitrogen and the acceptor in the H-bond with the amino

group. The binding energy (including ZPVE and BSSE) is 5.7,(5.3) kcal mol�1 at the B3LYP, (MP2) level. Other isomers areat least 2.7, (1.9) kcal mol�1 higher in energy. In these higherenergy isomers only one H-bond is formed. In the structuresportraited in 1b and 1d, respectively, water is the donor inan H-bond either with the aromatic N or with the aminogroup. In addition, the amino group may act as a donor in anH-bond with water (structures 1c). Structure 1b is the secondmost stable structure at both levels of theory. Isomers 1c and1d are the least stable structures.As was the case with 2AP�H2O iomers, the energetically

most stable structure of the 2AP�(H2O)2 cluster forms the max-imum number of H-bonds. Three isomers of 2AP�(H2O)2 withthree H-bonds are also depicted in Fig. 1. The isomers 2a and2b exhibit ‘daisy chain’ like H-bonds.23,24 This arrangementallows each molecule to act both as donor and acceptor,thereby forming more stable structures. The most stable iso-mer (2a) can be viewed as a water dimer forming H-bondswith the aromatic N and the amino group. This eight-mem-bered cyclic structure allows for almost perfectly linear H-bonding (with the angles FNr� � �HO , FNH1� � �O and FOH� � �O being176, 178 and 161�, respectively). The binding energy of twoH2O molecules with 2AP in this structure is 13.0 (11.6) kcalmol�1 (B3LYP, (MP2) level). Thus the binding energy is 4.1(2.2) kcal mol�1 larger than that of the next stable structure.In this spectrum the water dimer forms also two additionalH-bonds, however, both H-bonds are formed with the aminogroup (structure 2b). The resulting six-membered ring struc-ture is more strained than that of isomer 2a (with FN� � �HO ,FNH2� � �O and FOH� � �O angles being 151, 142 and 155�, respec-tively). The third isomer (2c) can be viewed as a combinationof the structures 1a and 1c, with one water molecule formingtwo H-bonds (with the aromatic N and the amino group)and one water being H-bonded as acceptor to the aminogroup. From the data, summarized in Table 2, it is evident thatthe binding energy of isomer 2c (8.0 (7.3) kcal mol�1) is almostexactly the sum of binding energies of isomers 1a and 1c (7.8(7.1) kcal mol�1).

3.2 R2PI spectra

The R2PI spectrum measured for the chromophore alone isdepicted in Fig. 2(a). The electronic origin band (000) is locatedat 33 466 cm�1, in agreement with the values in the literature.4,5

Fig. 2(b) shows the R2PI spectrum of the cluster 2AP�H2O.Compared to the isolated 2AP, its origin is red-shifted by919 cm�1. This very strong shift indicates that the cluster ismore stable in the S1 than in the S0 state. In their earlier workHager and Wallace reported a considerably smaller red shift of386 cm�1.5 They assigned mode 6a of the cluster as the origin(in our spectrum this band has a red shift of 381 cm�1 relativeto the origin of the monomer, close to the value of 386 cm�1).And beyond mode 6a of the cluster, the shape of our spectrumis similar with that reported by them.5 In addition, our IRdepletion spectra confirm beyond any doubt, that the bandat 32 547 cm�1 is really due to a 1:1 water cluster. When wescanned the wavelength further to the red, no other bands werefound.For microhydrated substituted benzenes red shifts in the

range of 100 to 300 cm�1 have been reported.8 The consider-ably larger red-shift observed in our experiment for 2AP�H2Ocompares well with that of 2-hydroxypyridine (2HP) withwater or methanol (666 or 822 cm�1, respectively).25 Thedipole moment of 2AP in isomer (1a) increases upon electronicexcitation from 1.99 D for the S0 state (at the HF/6-311G(d,p)level) to 2.11 D for the S1 state (at the CIS/6-311G(d,p) level).This increase and perhaps a stronger H-bond may rationalizethe additional stabilization in the excited state of the cluster.The R2PI spectrum of 2AP�(H2O)2 is shown in Fig. 2(c).

The origin band now appears at 32 126 cm�1. Hence the

516 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 5 – 5 2 1 T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4

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second water molecules induces an additional red shift of 421cm�1. Thus, the relative stabilization of the S1 state of 2APwith respect to its ground state induced by the first water islarger than that induced by the second water.Some vibrational bands of the aromatic chromophore exhi-

bit different frequencies in the cluster. For example, modes 6aand 6b, corresponding to in-plane ring deformations, in themonomer at 526 and 544 cm�1 are respectively blue-shiftedto 538 and 556 cm�1 in 2AP�H2O and to 553 and 572 cm�1

in 2AP�(H2O)2 . These differences are mainly due to the changein the reduced mass of the microsolvated chromophore.

3.3 Infrared ion depletion spectra of the clusters

3.3.1 2AP .H2O. Fig. 3(a) displays the IR dip spectrum of2AP�H2O measured by fixing the wavelength of the ionisinglaser to the 000 transition of this cluster. No other isomer isobserved for this cluster. When we fixed the UV wavelengthto higher energy transitions in the R2PI spectrum, the sameIR ion-dip spectra were observed (not shown). Moreover, in

a hole-burning spectrum, with the IR laser set to 3403 cm�1

and the UV laser being scanned, all bands in the R2PI spec-trum are depleted similarly. This result indicates the existenceof only one cluster species under these beam conditions. Thisobservation is also consistent with the results of the minimumenergy calculations reported above. The isomer 1a is at least2 kcal mol�1 more stable than other isomers. Assuminga Boltzmann distribution, the population of higher energyisomers is negligible at the experimental conditions.The lower part of Fig. 3(a) contains the spectrum of isomer

1a calculated at the B3LYP/6-311G(d,p) level of theory. Thevibrational bands in the IR-dip spectrum are summarized inTable 3, together with the calculated values of isomers 1a–1d.For isolated water, the symmetric (n1) and antisymmetric

(n3) OH stretching vibrations appear at 3657 and 3756 cm�1,respectively.26 For the 1:1 cluster these bands appear at 3403and 3717 cm�1. Due to the strong red shifts of the vibrationsin the cluster we may assume that the OH modes now exhibita more local character. The band at 3717 cm�1 is assigned tothe stretching vibration of the dangling OH group. It is red

Fig. 1 Calculated structural isomers of 2AP�H2O (1a–1d) and 2AP�(H2O)2 (2a–2c) obtained at the B3LYP/6-311G(d,p) level.

T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 5 – 5 2 1 517

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shifted by 39 cm�1 relative to the n3 band of free water. Thisfrequency shift for the free OH group is a secondary effectinduced by decoupling of OH stretching vibrations due tothe strong hydrogen bond of the donor OH group. For similartype clusters of A�H2O (A ¼ difluorobenzene, fluorobenzene,benzene, anisole or phenol), the corresponding bands are redshifted by 14, 14, 25, 28 and 8 cm�1, respectively.8 The largershift observed in 2AP�H2O indicates that water forms a muchstronger hydrogen bond with the aromatic nitrogen atom thanwith the other chromophores. According to the calculatedminimum energy conformation, water forms a cyclic structurewith 2AP containing two H-bonds. It donates a hydrogen tothe aromatic N and accepts a hydrogen from the amino group.This kind of cyclic structure is also found in 2- hydroxypyri-dine (2HP),25 and 2-phenylacetamide23 clusters with water,and in many similar clusters.The strong band at 3403 cm�1 is assigned to the H-bonded

OH stretching vibration. It exhibits a large red shift (254 cm�1)as compared with the n1 band of free water. This shift is typicalfor the stretching frequency of a OH group involved in astrong H-bond. The red shift is due to the weakening of theOH bond by an interaction with 2AP. This H-bonding inducedred shift is commonly rationalized by a transfer of partialcharge from the acceptor into the s* orbital of the donor.The magnitude of the frequency shift of the OH stretch is oftenused as a measure of the strength of the H-bond. Thus, thelarge frequency shift also gives evidence for a strong H-bond

with 2AP. This peculiarity is consistent with the relatively largeproton affinity of 2AP (947.2 kJ mol�1).27 It is also similar tothe cluster of 2HP with water, for which the correspondingband is red shifted by 214 cm�1.25

The difference between the relative intensities of the two OHstretch bands is another signature of the formation of an H-bond. For a free water molecule only the n3 band shows strongIR activity. In the spectrum of 2AP�H2O, the intensity of theH-bonded OH stretching band is much stronger than that ofthe free OH stretch, because of a reduction of symmetry ofthe H-bonded water. The two O–H bond lengths of waterare 0.980 and 0.963 A in the 1:1 cluster at the B3LYP/6-311G(d,p) level.In order to observe the shift in the vibrational frequency of

the amino group induced by the cluster formation, we firstsampled these vibrations for the isolated monomer. Since pre-dissociation is not possible in this case, depletion can only beinduced by depopulating the ground state by optical pumping.This practise renders IR induced ion depletion inefficient.However, the chromophore easily shows a depletion spectrumof its vibrations if a noble gas atom is bound to the p-system.The noble gas atom is then a scavenger or ‘‘ spy’’ of the IRabsorption. The infrared dissociation spectra of the phenol�ArAr and phenol�Kr clusters were measured by Fujii et al. Theyfound that the perturbation induced by the rare gas atoms isvery small (�2 cm�1) and the observed frequencies representthose of the monomer.28

The symmetric (n1) and antisymmetric (n3) stretching vibra-tion bands of the amino group of 2AP�Ar appear at 3439 and3546 cm�1, respectively.29 Calculations for the 1:1 cluster showthat the IR intensity of the free NH stretch of the amino groupis very weak, as depicted in the calculated spectrum. Thus theweak band at 3548 cm�1 is assigned to the free NH vibration.This band has almost the same position as the antisymmetricNH stretch band in an 2AP�Ar cluster. It is hence assignedto the free NH vibration. The band at 3315 cm�1 is assignedto the H-bonded NH stretch. It exhibits a red shift of 124cm�1 relative to the symmetric stretch in 2AP�Ar. The forma-tion of the H-bond induces the weakening of the NH bondtogether with an increase of the bond length from 1.009 to1.017 A.The unusual broad band at 3343 cm�1 is tentatively assigned

to the overtones of the bending vibrations of either water orthe amino group due to a Fermi resonance with the H-bonded

Table 1 Some important structural parameters of different isomers of 1:1 and 1:2 clusters calculated at the B3LYP/6-311G (d,p) level

2AP�H2O 2AP�(H2O)2

2AP 1a 1b 1c 1d 2a 2b 2c

RC–NH 1.382 1.368 1.378 1.369 1.401 1.359 1.397 1.357

RCN–H1a 1.009 1.017 1.008 1.007 1.012 1.023 1.012 1.015

RCN–H2b 1.008 1.006 1.007 1.012 1.012 1.006 1.019 1.011

RNr� � �OHc 1.940 1.964 1.808 1.927

RN� � �OH 2.110 2.003

RNH1� � �O 2.025 1.892 2.069

RNH2� � �O 2.029 2.067 2.021

ROH� � �O 1.772 1.848

FNr� � �OH 153.60 157.83 176.23 155.40

FN� � �OH 143.78 151.19

FNH1� � �O 147.66 178.29 145.98

FNH2� � �O 166.07 142.38 166.85

FOH� � �O 161.29 155.42

Dipole moment/D 1.946 2.385 4.117 4.224 1.389 2.483 1.294 5.026

a The bond length between nitrogen and H1 of the NH2 group adjacent to the ring nitrogen. b H2 is the second hydrogen in amino group. c Nr

represents nitrogen in the ring, N nitrogen in the amino group.

Table 2 Binding energies (kcal mol�1) calculated at the B3LYP/6-311G(d,p) and MP2/cc-pVTZ levels

2AP�H2O 2AP�(H2O)2

1a 1b 1c 1d 2a 2b 2c

ZPVE (B3LYP) 2.55 2.12 1.50 2.13 4.96 5.09 5.01

B3LYP/6-311G(d,p)

Ebind 12.68 8.54 7.30 6.97 26.50 22.14 20.00

EbindþZPVEþBSSE 5.70 2.97 2.12 1.04 13.00 8.91 8.03

MP2/cc-pVTZ

Ebind (MP2)a 10.85 7.95 5.62 6.94 22.43 20.03 16.60

EbindþZPVEþBSSE 5.32 3.40 1.78 2.26 11.59 9.39 7.27

a Single point MP2/cc-pVTZ energies calculated at the optimised B3LYP/

6-311G(d,p) structures.

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OH stretches. The calculated values of water and the aminobending modes are 1644 and 1638 cm�1 (scaled by a factorof 0.98), respectively. Similar overtone bands have beenobserved for other microhydrated aromatic molecules.8

Among the four bands, the two bands of free NH and OHvibration are weak at the high frequency side, which is consis-tent with the calculated spectrum. The other two bands,assigned as the vibration modes of the bonded OH and NH,are very strong in the measured spectrum. In the calculatedspectrum the bonded OH vibration band is very strong. How-ever, the bonded NH band is weak, which is different from theexperimental band. This may be attributed to the inaccuracy ofcalculated IR intensity.According to the IR-depletion spectrum, we may deduce

that only one species (isomer 1a) exists at the molecular beamconditions. Only isomer 1a shows a relatively large red-shiftfor both the NH and OH vibrations due to the formation ofH-bonds. Good qualitative agreement was found betweenthe calculated and experimentally determined vibrations of thisisomer. Isomers 1b and 1d show NH stretching vibrations verysimilar to those of isolated 2AP. Isomer 1c shows OH stretch-ing vibrations similar to those of isolated water. Thus thevibrational signature unambiguously reveals that only isomer1a is observed.

3.3.2 2AP�(H2O)2. The IR-dip spectrum of 2AP�(H2O)2 isshown in Fig. 3(b). The UV laser wavelength was again fixedto the 000 band of this cluster at 32 126 cm�1

Again only a single isomer of 2AP�(H2O)2 is observed. Wechecked this by using the hole-burning spectroscopy. Withthe IR laser set to 3343 cm�1 and the UV laser being scanned,all bands in the R2PI spectrum are depleted too much. It isclear that there is only one isomer under these conditions.Based on the theoretical results presented in section 3.3.1 weassign it to the calculated isomer 2a. Other isomers are at least2 kcal mol�1 higher in energy. Therefore, under the conditionsof our molecular beam isomer 2b and 2c are not produced ortheir amount is too small to be detected. The correspondingcalculated spectrum of the most stable cluster (2a) is shownin the lower trace of Fig. 3(b) for comparison. The observedvibrational bands and the calculated stretches of isomers 2a–2c are listed in Table 3. From the measured spectrum andthe calculated values, it is also easy to judge that the isomerexisted is not isomer 2c, which contained one free OHvibration band and the two bands of isolated water.It is apparent that the two bands at 3709 and 3719 cm�1 may

be assigned to two free OH stretches. For their assignment oneshould compare the spectrum of an isolated water dimer.There one water is the donor molecule, forming a hydrogenbond to the oxygen atom of the acceptor water molecule. Inthe stretches the latter are coupled similarly to those in isolatedwater, forming a symmetric and an antisymmetric mode andexhibiting only a weak shift. Thus the water dimer band at3745 cm�1 exhibits only a small red shift (11 cm�1) relativeto the n3 of the monomer. A second free OH stretch at 3735cm�1, which exhibits a red-shift of 21 cm�1, is howeverassigned to the free OH of the donating water molecule inthe dimer.30

The OH stretching vibration bands of 2AP�(H2O)2 exhibitshifts of 37 and 47 cm�1 relative to the n3 of free water. Hence,we may deduce that in the attached dimer both water mole-cules are proton donors. This conclusion is confirmed by thecalculations. The water dimer binds to the aromatic nitrogenand the amino group forming a ring structure. Therefore thetwo bands on the blue side of the spectrum are assigned tothe free OH stretches of the dangling OH groups of the watercluster.In the spectrum of 2AP�(H2O)2 the free NH stretch of the

amino group appears at 3536 cm�1, at a similar position as

Fig. 2 R2PI spectra of 2AP (a) and clusters of 2AP with water,2AP�H2O (b) and 2AP�(H2O)2 (c) in the vicinity of the vibrationlessS1 S0 transition.

Fig. 3 IR/R2PI spectra of 2AP�H2O (a) and 2AP�(H2O)2 (b). Thespectra were recorded with the UV laser fixed to the 000 transitions ofthe clusters. The lower inserts in (a) and (b) are the spectra calculatedat the B3LYP/6-311G(d,p) level, with the frequencies scaled by thefactor 0.98.

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in 2AP�H2O. The three strongly red shifted bands at 3408,3343 and 3224 cm�1 may be assigned to the stretching vibra-tions of OH and NH H-bonded donor groups forming thering. These three modes are effectively coupled in the cyclicstructure 2a. Such collective vibrations were also reported inmany other studies.31,32 The band at 3408 cm�1 mainly corre-sponds to a vibration with strong elongations of the H-bondedOH group in the water dimer. The band at 3343 cm�1 corres-ponds to a vibration with strong contributions of the localizedH-bonded NH stretch. In the 2AP�(H2O)2 cluster this band isexpected to have a larger red shift than that of 2AP�H2Obecause the H-bond between the amino group and water isstronger due to a reduced geometric strain in the ring withtwo water molecules. Although this ring stretching vibrationis formed by all the three groups forming the ring, the indi-vidual vibrations do not show the same phase. The lowestfrequency at 3224 cm�1 is assigned to a ring stretchingvibration with all groups vibrating with similar phase. Com-pared to the corresponding ring mode in 2AP�H2O, itsfrequency exhibits a considerably enhanced red shift.The bandwidths of the H-bonded OH and NH stretching

vibrations are much larger than those of the free OH andNH stretching vibrations. Huang et al.33 studied the lifetimeand line width of the different OH stretching vibrational levelsin the water dimer by high resolution spectroscopy. The life-time of the H-bonded OH stretch vibrational level of the donorwater is considerably shorter than that of the free OH stretch.Thus, the relaxation and predissociation rate depend stronglyon the coupling of the intramolecular vibration to the intermo-lecular vibrational modes. The coupling of the H-bonded OHstretching vibration with other lower frequency intermolecularvibrations is much stronger than that of the free OH stretchingvibration. This causes a decrease of the lifetime and, thus, anincrease of the line width. In 2AP�(H2O)2 the H-bonded OHand NH stretches have stronger coupling to the intermolecularvibrations, and thus predissociation is faster than when a clus-ter is vibrationally excited at the free OH and NH groups,resulting in shorter lifetimes and wider line width.The discrepancies between observed and calculated band

intensities need some explanation. While the observed intensi-ties of the H-bonded OH and NH stretching vibrations aredominant both in the experimental and calculated spectra,the relative intensities of the observed free OH and NH vibra-tions are enhanced in comparison to the calculated ones. It isknown that comparing relative intensities of calculated andmeasured vibrations one has to take into account manysources of discrepancies. First of all, intensities are difficultto calculate and depend on the level of theory, size of the basisset, anharmonicity and coupling of the calculated vibrationswith other vibrations etc. In addition in the experiment thelaser intensity is often adjusted to get acceptable signalintensity for the weakest bands. At such laser intensity levelstronger transition are often partly or fully saturated. This

leads to a relative enhancement of the weak bands as probablyin the case of the present spectra. Normalization of the spectrato the laser intensity cannot compensate this effect. In our casethe spectra have been normalized to a laser output showing anearly flat wavelength dependence, except for the very narrowhole in the transmission curve of the LiNbO3 crystal, whichdo not overlap with any of the measured bands. Therefore dis-crepancies in the relative intensities should not be overinter-preted, as long as the intensity relations agree more or lessqualitatively.The bands at 3282 and 3287 cm�1 are nearly overlapping,

and are assigned to Fermi-resonance enhanced overtones ofthe OH and NH bending vibrations, in good agreement withthe calculated values of the OH bending vibration of the twowater molecules (1643 and 1648 cm�1) and the amino group(1621 cm�1).Since the calculated vibrational spectra fit qualitatively quite

well with the measured one we may confidentially proceed to dis-cuss the calculated binding energies and structural parameters.The binding energies (including ZPVE and BSSE correc-

tions) are 5.6 and 13.0 kcal mol�1, respectively for the 1:1(1a) and the 1:2 (2a) clusters when calculated at the B3LYP/6-311G(d,p), and 5.32 and 11.59 kcal mol�1, respectively,when calculated at the MP2/cc-pVTZ level. It should be notedthat while in the 1:1 cluster two H-bonds are formed and in the1:2 complex an additional H-bond is added, the intermolecularbinding energy of 2AP�(H2O)2 is more than twice as large asthat of 2AP�H2O.After forming a cluster with water, the N–H1 bond of the

amino group is elongated from 1.009 to 1.017 and 1.023 A inthe 1:1 and the 1:2 cluster, respectively. This gives clear evidencethat water forms not only an H-bond with aromatic nitrogen,but also with the amino group. For the 1:1 complex (1a), thisH-bond is not very strong, because the NH group does notform a linear H-bond with the oxygen of water, as seen inFig. 1 (1a). The calculated value of (FNH� � �O) in the 1:1 cluster(1a) changes however from 147.66 to 178.29� in the 1:2 cluster(2a). Hence, with two water molecules it is easier to form an H-bond with the amino group than only one water molecule,resulting in an enhanced bond strength in the 1:2 complex. Thisis further reflected in the lengths of the other H-bonds calcu-lated for the 1a and 2a clusters. The length of the H-bondbetween the ring nitrogen atom and water is 1.940 A for cluster1a and 1.808 A for cluster 2a. Similarly the H-bond betweenthe amino-group hydrogen and water is shortened from 2.025to 1.892 A for clusters 1a and 2a, respectively.

4 Conclusions

The R2PI spectra of 2AP�H2O and 2AP�(H2O)2 have beenmeasured. The origins of the S1 S0 transition is shifted by919 and 1340 cm�1, respectively, relative to that of the isolatedchromophore. The electronic excitation induces a change of

Table 3 Observed vibrational bands in the IR-dip spectra and frequencies calculated at the B3LYP/6-311G (d,p) level of theory in cm�1

2AP�H2O 2AP�(H2O)2

Calculated Calculated

Observed 1a 1b 1c 1d Observed 2a 2b 2c

NHbonded (or n1) 3315 3375 (33)a 3510 (42) 3460 (240) 3464 (25) 3343 3294 (1371) 3377 (73) 3388 (145)

NHfree (or n3) 3548 3608 (59) 3621 (29) 3606 (92) 3562 (26) 3536 3607 (43) 3543 (69) 3547 (396)

OHbonded (or n1) 3403 3446 (853) 3551 (520) 3737 (15) 3630 (179) 3242 3207 (614) 3460 (358) 3435 (806)

3408 3409 (578) 3511 (585) 3739 (17)

OHfree (or n3) 3717 3780 (44) 3787 (43) 3835 (55) 3798 (49) 3709 3784 (50) 3790 (44) 3779 (38)

3719 3787 (31) 3805 (52) 3838 (56)

a The values in bracket are calculated IR intensities.

520 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 5 – 5 2 1 T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4

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the dipole moment, and probably of the H-bonds, thusstrengthening the interaction with water in the electronicallyexcited state.For 2AP�H2O, the H-bonded OH stretching band of water

at 3403 cm�1 shows a large red shift (254 cm�1) and is veryintense as compared to the n1 band of free water. Both theincrease of the intensity and the large red shift indicate thatwater forms a strong H-bond with the aromatic N. At the sametime water is an acceptor and forms another hydrogen bondwith the amino group. Other 1:1 cluster species are notobserved.In the IR depletion spectrum of 2AP�(H2O)2 , the three nar-

row bands at the high frequency side are assigned to two freeOH and one free NH stretching vibrations. Another threemuch broader and strongly red shifted bands are due to thecoupling of H-bonded OH and NH stretching vibrations inthe ring. Because of the less strained geometry of the clusterof 2AP with two water molecules as compared with one watermolecule, much stronger H-bonds are formed correspondingto a considerable increase of the binding energy per H-bond.The clusters of 2AP and water include several different H-

bonds (OH� � �N, OH� � �O and NH� � �O), which may well bestudied by IR/R2PI vibrational spectroscopy in combinationwith ab initio calculation. Further studies on clusters contain-ing a heterocyclic molecule and solvent molecules will help usto get a deeper insight into the interaction between a biomole-cule and its surrounding solvent.29

Acknowledgements

R. W. gratefully acknowledges a stipend of the Alexander vonHumboldt Foundation, and P. Nachtigall a guest professor-ship of the Hertie Foundation enabling their stay. P. N. wishesto thank the Czech Ministry of Education for support for theCenter for Complex Molecular Systems and Biomolecules(Grant No. LN00A032). B. B. wishes to thank the Fondsder Chemischen Industrie for its support. The help by Dr B.Reimann, Dr H.-D. Barth, Dr Ch. Riehn and S. Vaupel is alsogratefully acknowledged.

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T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4 P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 1 5 – 5 2 1 521

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