Electron attachment products of methylene chloride in solid argon: an experimental and quantum chemical IR spectroscopic study

Chemical Physics

ELSEVIER Chemical Physics 214 (1997) 321-328

Electron attachment products of methylene chloride in solid argon: an experimental and quantum chemical IR spectroscopic

study

Andreas Richter, Hermann Meyer, Thorsten Kausche, Thomas Mi~ller, Walter Sporleder, Armin Schweig *

Fachbereich Physikalische Chemie und Zentrum f~r Materialwissenschaften der Philipps-Universit~t, Hans-Meerwein-Strasse, D-35032 Marburg, Germany

Received 16 September 1996

Abstract

The nature and structure of electron attachment products in the photoinduced electron transfer reaction DMA + CH2C12 hv

DMA ÷ +(electron attachment products) in solid argon has been studied by FTIR spectroscopy (DMA = N,N- dimethylaniline, hu = 4.88 eV Hg lamp radiation) along with extensive ab initio calculations. Two attachment products have been identified: the radical anion of CH2C12 and the dichloromethane anion (HCCI~-). The radical anion turns out to have a C j hydrogen bridged structure (C1HCH . . . . CI-). This structure as well as the species HCCI~- have not been detected before.

1. In t roduc t ion

In a study on the resonant two-photon ionization (TPI) of phenol in methylene chloride (CH2C12) doped solid argon using 4.88 eV photons we em- ployed a kinetic model which qualitatively allows to understand the observed incomplete running of the ionization reaction [1]. The model includes the as- sumption that the freed photoelectron is captured by CH2Cl 2 to form the stable complex radical anion C H E C I - . . C1-. At that time no experimental or theoretical evidence for the existence of this complex, its structure and spectroscopic properties was available. In order to clarify the situation we under-

* Corresponding author.

took the present experimental and quantum chemical IR (infrared) investigation.

To our knowledge a stable form of the methylene chloride anion has only been detected in a %, irradiated, tetrahydropyran doped methylene chloride glass [2]. From an ESR (electron spin resonance) spectroscopic investigation Czv symmetry was deduced for the anion. Ab initio calculations (in minimal basis set quality and without taking into account electron correlation) suggested prolongations of the neutral molecule CCI bond lengths by ~ 20%. Moreover, the methylene chloride radical anion appears to be known only as a temporary, in the gas phase observ- able system [3,4].

In the present work we use N,N-dimethylaniline (DMA) instead of phenol [1] as donor in the photoin-

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322 A. Richter et al. / Chemical Physics 214 (1997) 321-328

duced electron transfer (PET) reaction since the radical cation DMA "÷ is obviously formed without by- products and in a relatively high yield. In this way, high concentrations of reduction product(s) of CH2C12 are anticipated.

2. Experimental

Matrix experiments were carried out at 13 K employing an Air Products Displex CSW 202 closed-cycle helium cryostat. Matrix mixtures of Ar /CH2C12/DMA in a ratio of 1000:2:1 were externally prepared using an MKS Baratron and a piezoelectric vacuum gauge and deposited on a KBr or sapphire plate for recording FTIR (Fourier transform IR) and UV/VIS (ultraviolet/visible) absorption spectra, respectively. The matrices were irradiated through suprasil windows and the spectra taken through KBr (FTIR) and suprasil (UV/VIS) windows. Irradiation was accomplished using a GrS_ntzel low pressure mercury lamp operated at a lamp cur- rent of 200 mA and filtered through a Schott UG5 bandpass filter or an IF 254 nm interference filter. The matrices were irradiated for 3 h.

The UV/VIS absorption spectra were run on a Shimadzu UV-260 spectrometer (spectral range 190-900 nm) at a spectral resolution of 0.8 nm. The signal to noise (S /N) ratio was improved by averag- ing three spectra in each case. The FTIR spectra were recorded on a Bruker IFS 120 HR spectrometer (best attainable resolution: 0.00165 cm -~ ) at a resolution of 0.5 cm- i,

The UV/VIS spectrum of DMA was obtained by subtracting the matrix plate spectrum (background spectrum) from the matrix spectrum recorded before irradiation. The corresponding spectrum of DMA "÷ is represented by the difference between the (background corrected) matrix spectrum obtained after irradiation and the weighted DMA spectrum. The FFIR spectrum was obtained by subtracting the matrix spectrum taken before irradiation from the matrix spectrum taken after irradiation.

CH2C12 and DMA were bought from Fluka puriss. p.a. and were used without further purification. Ar was 99.98% pure. CH2CI 2 and DMA were thor- oughly degassed immediately before they were used.

3. Computational details

The ab initio calculations of the molecular structures and IR spectra were performed employing the GAUSSIAN 92 [5] program system. DMA + was treated on the ROHF (restricted open-shell Hartree- Fock) level in a 6-31G * * basis [6], all other species with electron correlation included on the UMP2 (unrestricted HF with Moller-Plesset 2nd order perturbation theory [7]) level in the 6-31 + + G * * {CI: 6-31 + + CA3d2f)} basis, i.e. in a 6-31 + + G** basis [6], for which, instead of the provided set of d functions, 3 sets of d and 2 sets of f functions (according to Ref. [8], determination of the f function exponential coefficients in analogy to sulphur) were used. Matrix effects were not taken into account.

4. Results and discussion

Under the aforementioned conditions (cf. Section 2) ~ 50% of DMA reacts to the radical cation DMA ÷ according to the UV/VIS absorption spectra. By-products as, e.g., the N-methylaniline radical which was observed in organic glasses at 77 K [9] do

~ . /nm 700 500 300

I i I I I

I A = 0 5

A x 112o i

~ . _ . . ~

1.s ' £o 21s 3;0 a~5 41o '41s s~o AE / eV

Fig. 1. UV/V IS absorption spectra (absorbancc A versus excita- tion cnergy A E and wavelength A) of N,N-dimethylaniline (DMA; - - - ) and N,N-dimethylaniline radical anion (DMA+; ~ ) in a methylcnc chloride (CHzCI 2) doped argon matrix. The spectra are difference spectra (for details see the experimental part in tcx0.

A. Richter et a l . / Chemical Physics 214 (1997) 321-328 323

I I . • I I

A

. . . . . . . . . . . . , . . . . . . . . . I I ~ . . . . . . . . . . . . . . , . . . . . . . . I I . . . . . . 3100 3000 2900 2800 950 900 850 800 550 500

~/cm-1

Fig. 2. FFIR absorption spectrum (absorbance A versus wavenumber 9) of N,N-dimethylaniline (DMA; bands marked with O), N,N-dimethylaniline radical cation (DMA+; bands marked with O), methylene chloride (CH2CI2; band marked with n), methylene chloride radical anion (CIHCH • -- CI-; 1; bands marked with II), and dichloromethane anion (CHCI]; 3; bands marked with X). The spectrum is a difference spectrum (for details see the experimental part of text).

not occur in the irradiated argon matrix as inferred from the UV/VIS absorption spectra. Moreover these spectra do not present any evidence for reduction products of CH2C12 (measured range: 1.4 to 6.5 eV).

The U V / V I S absorption spectrum of DMA ÷ is shown in Fig. 1. The weak band in the region of 770 to 540 nm (1.6 to 2.3 eV) and the partially vibra- tionally resolved band at 480 nm (2.58 eV) agree

very well with the known bands of DMA + (obtained after ~/[10] or UV [9] irradiation of DMA in organic glasses). Our spectrum displays an additional intense band at 4.4 eV (280 nm) which, according to the photoelectron spectrum of DMA [11], is expected to occur at 4.3 eV (as based on vertical ionization energies).

Fig. 2 displays three sections of the FTIR (dif-

Table 1 Wavenumbers ~ (in c m - t ), integrated absorption coefficients ,~¢ (in km too l - t ) and relative intensities lre I (relative to the most intense band) for the IR absorption bands of dimethylanil ine (DMA; bands marked with C)) and dimethylanil ine radical cation (DMA+; bands

marked with 0 )

Molecule Calculated a Measured c

b 5at" lrel liquid argon matrix

lrel "~ lrel

DMA

DMA .+ 969 35 0.038 942 5 0.005 9OO 9 0.010

803 72 0.077 778 d

783 68 0.073 766 929 e 1

992 m 945 m 864 w

992.1 m 0 949 .5 /947 .9 m 0 8 6 1 . 6 / 8 6 0 . 4 / 8 5 6 . 7 w © 978.2 w • 9 2 0 . 4 / 9 1 9 . 0 / 9 1 7 . 2 w • 909 .2 /907 .3 w • 792.7 m •

785.1 w • 779.1 m •

776 .1 /775 .2 s •

a Restricted open-shell Har t ree-Fock (ROHF) calculations in a 6-31G * * basis. b Calculated wavenumbers mult ipl ied (scaled) with a factor of 0.89. c This work.

d Assumed to be an overtone vibration enhanced by Fermi resonance (see tex0. e The unusually high value appears to be an artifact of the GAUSSIAN 92 calculation.

324 A. Richter et a l . / Chemical Physics 214 (1997) 321-328

ference) spectrum. The V (wavenumber) regions were chosen such as to contain the bands which are essential for the identification of the reduction products of CH2CI 2. All bands which are relevant to the identification of educts and products are marked with various symbols (O , U/, O, • and ×). Among the many educt CH bands which could be expected in the region of 3150 to 2700 cm- ~ only two bands at 2856 and 2821 cm- I can be identified (due to an unfavourable S / N ratio). These bands (©) correspond to the bands of liquid DMA at 2848 and 2804 cm-~ (this work; for band shifts see below). The sharp bands (" l ines") in the range 570-500 cm- are due to water [12]. Among the educt bands which can be recognized in the middle ~ range (1000-760 cm -~ ) the band at 894 cm -1 (r-l) belongs to CH2C1 z [13]. The remaining educt bands (O) agree well with the corresponding bands of liquid DMA at room temperature (cf. Table 1); the appearing slight shifts and splittings result from matrix effects [14].

In order to assign the remaining IR bands to products of DMA or CH2C! 2 we investigated analo- gous PET reactions substituting N, N, N', N'-tetra- methylphenylenediamine for DMA and/or substituting carbon tetrachloride (CCI 4) for CH2C12. A com- parison of the resulting spectra unequivocally shows which of the marked bands of Fig. 2 belong to products of DMA ( O ) and CH2CI 2 ( • and ×) , respectively.

The product bands ( O ) belonging to DMA excel- lently correlate with the calculated ROHF (6-31G * * basis set) fundamental vibrations (wavenumbers scaled with the usual factor of 0.89) of DMA + (see Table 1) with one exception. The weak band of 785.1 cm- l is probably an overtone vibrational band whose intensity is enhanced by Fermi resonance and to which the value of 389 cm- I among the calculated fundamental wavenumbers could belong. The interpretation that all DMA product bands exclu- sively belong to DMA + is in accord with the UV/VIS spectroscopic findings.

Among the five bands which have been identified as product bands of CH2Cl 2 ( • and × ) there are three prominent bands in the CH stretching region (cf. Fig. 2) at 3108, 2882 and 2764 cm - t . Corre- sponding bands were also observed for Ar/CH2Cl 2 samples subject to VUV (vacuum UV) radiation during condensation at 10 K [15]. In this paper the

band at 2764 cm-~ had not been assigned; the two other bands were ascribed to the complex (CH2CI+)C1. The formation of such a complex must be excluded under the experimental conditions of the present work, since the photon energy of 4.88 eV is too low to bring about one-photon ionization or resonant two-photon ionization of CH2C12; for a non-resonant TPI the light intensity of a Hg lamp is much too low. Accordingly, the bands at 2882 and 3108 cm - j cannot be interpreted as done in Ref. [15]. Also the photoisomerization of CH2CI 2 to iso- dichloromethane as observed with light of 6.42 and 6.70 eV in an argon matrix [16] must be excluded, in our investigations, for energetic reasons; in addition the bands at 763.2, 772.6 and 958.5 cm-1 which were assigned to iso-dichloromethane are absent in our spectrum [ 16].

The aforementioned three CH bands indicate that (at least) two different species are formed from CH2C12. Therefore we have a priori to consider all formally possible anionic and neutral dissociation products of CH2C12, in addition to the anticipated radical ion CHeC1 ~. For some of these species measured IR data are available [17-20]; in the other cases the needed IR data were calculated. Thus it could be shown, that two of the three CH bands must be assigned to the radical anion of CH2C12 whereas the third band obviously belongs to the CHCI~

Table 2 Calculated geometries (bond lengths in A, bond and dihedral angles in deg) of the methylene chloride radical anion (1), the monochloromethane radical (2) and the dichloromethane anion (3) a

Molecule Point Bond length Bond and dihedral angle group

C I H C H ' C I -

(1)

CH2CI (2)

CHCI~ (3)

C I C-C11 1.725 H I - C - H 2 123.0 C-H I 1.087 H I - C - C I 1 117.7 C-H 2 1.078 H I - C - C I 2 12.2

H I - c I 2 2.405 H 2 - C - C I 2 110.9 H t - C - C 1 1 - H 2 153.9

C s C-CI 1.701 H - C - H 122.9 C-H 1.076 H - C - C I - H 157.7

C s C-C1 1.891 CI -C-CI 104.0 C-H 1.103 CI -C-C1-H 99.6

a Unrestricted HF with Moller-Plesset 2nd order perturbation (UMP2) calculations in a 6-31 + + G * * {Cl: 6-31 + +G(3d2f)} basis.

A. Richter et a l . / Chemical Physics 214 (1997) 321-328 325

CIHCH'...CI- CH2CI" CHCI2- (C,) (Cs) (Cs)

1 2 3

anion. In full accord with this surprising result is the interpretation of the remaining two bands in the CC1 stretching region (at 794 and 532 cm- l ) .

According to the calculations the radical anion of CH2CI 2 can formally be described as a H (hydrogen) bridged structure CIHCH . . . . CI- (1) (point group C l) shown above (for the structural data con- sult Table 2). The H-bridged C1 atom is quite distant from the CH2CI rest (bond length H • • • C I - = 2.4 • ~) and carries the main portion of the negative charge. The geometry of the CHzC1 rest comes quite close to a free CH2C1 radical (2) (cf. Table 2). Regarding the geometric structures the dissociation of 1 to 2 and C1- is already gradually developed. The calculated dissociation energy (sum of the energies of 2 and C1- diminished by the energy of 1) amounts to 0.39 eV (38 kJ mol-~). This (certainly not very reliable) value suggests that 1 could, under favourable conditions, also be stable in the gas phase (i.e. without the forces exerted by the matrix cage). It is to be noted that, according to the calculations, 1 should also be stable relative to the system CH2C12 + e - (the total energy of 1 is lower by 0.30 eV (29 kJ mol -~) than that of the CH2C12 molecule. This value represents the adiabatic electron affinity of CH2C12; the measured vertical electron affinity amounts to - 1.23 eV [4].

The computed IR vibrational wavenumbers and intensities for 1 as well as for CH2C12, CH2C1 (2) and CHCI~- (3) are listed in Table 3. The wavenumbers were scaled with 0.95 for all systems. This value which was occasionally used for calculations including electron correlation proved true also for the chlorine compounds studied in the present work. For the test example CH2C12 (Table 3), an average value of 0.955 was obtained for the scale factor

(using the experimental data of CH2C12 in an Ar matrix). Furthermore the example suggests that also the IR intensities are of a useful quality.

The calculated spectrum of 1 exhibits, as the most prominent band, a very strong band at 2987 cm- and two strong bands at 3198 and 769 cm-~. These data as well as the corresponding intensities correlate reasonably well with the bands in the measured spectrum at 2882, 3108 and 797 cm -1. The compari- son between theory and experiment indicates that the calculated wavenumbers of the CH stretching vibrations are too high by ~ 100 cm -1. In this context it is to be noted that, also in the corresponding case of CH2C1 e, the wavenumbers are too high (by ~ 50 e r a - 1).

The high intensity of the CH stretching bands is characteristic of the IR absorptions of 1 which might be surprising regarding the weak absorptions of the corresponding vibrations of CH2Cl 2. However similar results were obtained for the halomethanes CHC13, CHC12Br and CHCl2J and the corresponding radical anions (generated in photoionization experiments in an argon matrix). The high intensity of the CH stretching band was explained with the formation of a H-bridge in the anion [22].

The C~ structure postulated for the methylene chloride radical anion in the present work strongly deviates from the C2v structure derived for this anion in a ",/ irradiated methylene chloride glass [2]. Our quantum chemical calculations which were performed on a relatively high level were not indicative of a C2v structure. Thus the C2v structure found in Ref. [2] points to special microscopic effects in the glassy environment.

The CH2CI radical (2) which is anticipated (besides Cl- ) , if 1 dissociates, is obviously not formed under our experimental conditions. IR bands measured for 2 (generated by VUV photolysis of CH3C1 and CH2Cl 2 in an argon matrix) [21] and confirmed by our calculations (see Table 3) do not occur in our spectrum. It is important here to point to the strong changes in the IR spectra (in particular in the intensities, remember the "H-bridge effect" mentioned above) when going from 1 to 2 (cf, Table 3) al- though the geometric structures of both species are quite similar (cf. Table 2).

We assign the last two bands ( × ) of the IR spectrum (at 2764 and 532 cm -~) to the anion

326 A. Richter et al. / Chemical Physics 214 (1997) 321-328

Table 3 Wavenumbers 9 (in cm- t ), integrated absorption coefficients ,w' (in km mol- t ) and relative intensities [rel (relative to the most intense band) for the IR absorption bands of CH2CI 2 (band seen in the spectrum marked with []), CIHCH . . . CI- (1; bands discernible in the spectrum marked with • ) , CH2CI (2) and CHCI~ (3; bands visible in the spectrum marked with × )

Molecule Vibrational mode Calculated a Measured c

b 5~¢ lrel P lrel

CH2CI 2 Vas (CH 2) 3111 0.5 0.004 3059 w v s (CH 2) 3026 5 0.036 2993 w

(CH 2) 1432 0.05 0.000 1455 vw w (CH 2) 1237 51 0.397 1265 s r (CH 2) 1132 0 0 p (CH 2) 880 1 0.008 894 w [] Vas (CCI 2) 745 128 1 747 vs v s (CCI 2) 701 12 0.096 710 s 6 (CCI 2) 275 0.5 0.004

C1HCH . . . . C1- (1) Vas (CH 2) 3194 76 0.279 3108 0.20 • v s (CH 2) 2986 273 1 2882 1 •

(CH 2) 1396 15 0.053 p (CH 2) 986 15 0.053 vas (CCI z) 768 72 0.265 797 0.08 • to (CH 2) 644 39 0.145 ~" (CH 2) 195 2 0.007 d 127 31 0.114 d 54 7 0.027

CH2CI (2) Vas (CH 2) 3252 0.09 0.002 v s (CH 2) 3103 9 0.167 3 (CH 2) 1390 14 0.254 1391 wm p (CH 2) 982 0.007 0.000 p (CC1) 814 41 0.732 827 s o~ (CH 2) 459 46 1 402 s

CHCI] (3) v (CH) 2864 77 0.473 2764 × (HCC1) 1142 7 0.045

w (CCI 2) 985 2 0.010 gas (CCIz) 517 17 0.106 v s (CCI 2) 495 164 1 532 X

(CC12) 231 3 0.016

a UMP2 calculations in a 6-31 + + G * * {CI: 6-31 + + G(3d2f)} basis. b Multiplied (scaled) with a factor of 0.95. c In an argon matrix; CH2CI 2 data form Ref. [13]; data of 2 from Ref. [21], data of 1 and 3 from the present work. a The calculations do not allow the assignments in these cases.

C H C I ~ (3). Our obs e r va t i on (cf. CH2C12 and

C1HCH . . . . C i - above) that theory pred ic t s the C H

s t re tch ing v ib ra t iona l w a v e n u m b e r s a bi t too h igh

can be also m a d e here . In the p resen t case, h o w e v e r ,

the h igh in tens i ty o f this v ib ra t iona l b a n d c a n n o t be

exp l a ined wi th the f o r m a t i o n of a H-br idge . Accord -

ing to our ca lcu la t ions and in a g r e e m e n t wi th p rev i -

ous s t ructural da ta der ived f rom M P 2 / 6 - 3 1 + +

G(d ,p ) ca lcu la t ions [23], the g round state o f 3 is a

s ing le t s tate wi th a py ramida l geome t ry (po in t g roup

C s) (geomet r i ca l da ta in Tab le 2). The suppos i t i on

tha t a H - b r i d g e d i somer o f 3, in ana logy to 1, cou ld

be r e l evan t to the in te rp re ta t ion o f the spec t rum is

no t real is t ic with regard to the ca lcu la ted total ener-

gies.

A l r eady in an ear l ier paper , IR b a n d s h a v e b e e n

asc r ibed to 3, n a m e l y at 2720 , 2498, 1271 and 838

c m - ~ [24]. 3 had b e e n pos tu la t ed as a p roduc t o f the

in te rac t ion of an A r / c h l o r o f o r m (CHCI 3) sample

wi th the pe r iphe ry o f a m i c r o w a v e d i scharge t h rough

argon. A c c o r d i n g to Refs. [22,25], h o w e v e r , the

a f o r e m e n t i o n e d b a n d s shou ld be a s s igned to the hy-

A. Richter et al. / Chemical Physics 214 (1997) 321-328 327

drogen-bonded species C1- . . . HCCI 2. Our investigations confirm that these bands do not belong to CHCI~- (see above). To our knowledge this species has never been detected before the present study.

The question about the way on which 3 is formed under our experimental conditions cannot be clearly answered within the frame of the present investigation. One might speculate about the photolytic gener- ation of 3 from 1 (available photon energy 4.88 eV; however it is to be noted that the H atom must not only be split off the molecule but must also escape from the matrix cage (energy needed for this process at least ~ 1 eV [26])) or could imagine that high energy photoelectrons (from DMA) allow for an immediate CH bond breaking process in the vertical methylene chloride radical anion (geometry of the neutral molecule).

5. Conclusions

The electron attachment products of the acceptor methylene chloride (CH2C12) due to photoinduced electron transfer (using the 4.88 eV photons of a Hg lamp) from the donor dimethylaniline (DMA) were investigated by FTIR spectroscopy in solid argon, including extensive ab initio calculations to interpret the experimental results. The following conclusions are drawn.

(1) As anticipated methylene chloride radical anion is formed; it has the structure CIHCH . - - CI- (1, point group C~) which can be interpreted as a very loose hydrogen bonded complex of a chloromethyl radical (CH2CI') and a chloride ion (C1-). The calculated H • • • C1- distance amounts to ~ 2.4 ,~.

(2) The structure of the anion 1 formed in solid argon is basically different from the structure of the corresponding ion (point group C2v, same as the parent molecule) obtained in a tetrahydropyran doped methylene chloride glass after ~/ irradiation.

(3) The calculated molecular energies suggest that 1 is not only a stable species in solid argon (due to the cage effect) but possibly also in the gas phase (and that with respect to the dissociation to 2 and C1- as well as with respect to electron detachment).

(4) The CH stretching bands of 1 (at 3108 and 2882 cm -~) are very intense compared to the corre-

sponding bands of CH2CI 2 which may be due - similarly to the known cases of the trihalogenated methane radical ions - to the H-bridge.

(5) Besides 1 the dichloromethane anion CHCI~ (3) is produced. It has a singlet ground state with a pyramidal structure (point group C s) and exhibits a CH stretching band in the IR spectrum at 2764 cm- with a surprisingly high intensity. As distinguished from 1 it is not obvious how 3 is formed.

(6) Bands at 3108 and 2882 cm -~ observed for Ar/CH2C12 samples subject to VUV (vacuum UV) radiation during condensation to a matrix are very likely not due to the complex cation (CH2CI+)C1 as supposed in Ref. [15] but, as shown in the present paper, to the radical ion 1.

(7) Bands at 2720, 2498, 1271 and 838 cm -~ were ascribed to CHC12 (3) in Ref. [24] where 3 was postulated as a product of the interaction of an Ar/chloroform (CHC13) sample with the periphery of a microwave discharge through argon. According to the present investigation, instead of these bands, the bands at 2764 and 532 cm- 1 belong to 3. To our knowledge this species has never been detected before.

Acknowledgements

This work was supported by the Fonds der Chemischen Industrie. The calculations were carried out on the SNI $400/40 computer of the Rechenzen- trum der Technischen Hochschule Darmstadt. We are very grateful to Professor Manfred Winnewisser and his staff for placing the Fourier transform spectrometer at our disposal as well as for the help of handling the spectrometer and for many fruitful dis- cussions.

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Electron attachment products of methylene chloride in solid argon: an experimental and quantum chemical IR spectroscopic study