J. Phys. Chem. 1982, 86, 3374-3380
3374
hydrogen by deuterium supports the 3 a ~ assignment * given in Table I.' The 0' band in PCB probably corresponds to the 3na* So O2 band in PCB-d' since there is a -60-cm-l blue shift upon deuteration. Accordingly, the O2band observed in PCB aeem to have no counterpart in PCB-dl. However, a closer examination of the phosphorescence spectra of PCB-d1 suggests in fact that one of the weak bands observed near the onset of the spectra (Figure 2) corresponds to this band.
-
Phosphorescence Excitation Spectra The phosphorescence excitation spectra clearly supports the multisite mechanism and explains the different phosphorescence decays observed for O2 and 03.The excitation spectra (shown in Figures 5 and 6) were obtained by separately monitoring the intensity of the strong phosphorescence lines VI, V2, and Vs. Excitation into either the short-lived V1 or V2 of PCB (Figure 5A or 5B) or V2 of PCB-dl gives the typical vibronic contour corresponding to an na* transition, i.e., an (7)S. H.Hankin, 0. S. Khalil, and L. Goodman, Chem. Phys. Lett., 63,11 (1979).
intense origin band and virtually no vibronic activity for the first 1000 cm-' of the spectra.6 Excitation into the long-lived V3 or PCB (Figure 5C) or PCB-dl (Figure 6B) leads to a spectrum characterized by profuse vibronic structure and a relatively weak origin band. This kind of contour is typical of a aa* transition involved in vibronic coupling to a strong na*
Conclusion The independence of excitation spectra and the different phosphorescence components together with the different associated phosphorescence decay times lead to an unambiguous conclusion: the multicomponent emissions cannot originate from different enery levels in the same molecule. Instead we have demonstrated that the emissions can only be due to different sites. Acknowledgment. Supported by a grant from the Scientific Arm of the North Atlantic Treaty Organization. (8)J. Olmsted, 111, and M. A. El-Sayed, J. Mol. Spectrosc., 40,71 (1971). (9)0.S.Khalil, S. W. Hankin, and L. Goodman, Chem. Phys. Lett. 47,2026 (1977).
Fourler Transform Infrared Spectra of the C2H2-HX and C2HX-HX Hydrogen-Bonded Complexes in Solid Argon Lester Andrews," Gary L. Johnson, and Benuel J. Kelsall LJepartmnt of Ctmmktty, Univershy of Virginia, Charlottesvih, Virginia 22901 (Received February 1, 1982; In Final Form: April 19, 1982)
Fourier transform infrared spectra have been observed for hydrogen-bonded A complexes C2H2-HX and C2HX-HX produced by HX addition to C2H2and by vacuum-W photolysis of vinyl halides and dihaloethylenes. The C2H2-HXcomplexes were characterized by a strong substantially displaced H-X submolecule fundamental v5, by slightly displaced vzc, usc, vqC,and vgC modes of the C2H2submolecule, and by split components of the strong HF librational mode, vI. The orientation of the HX ligand in the "T-shaped" C2H2-HXcomplex is responsible for lifting degeneracy in the usc and (v4 + vs)c modes and for making vZcand one component of vqC infrared active. Vibrational data are also presented for several C,HX-HX complexes.
Introduction Infrared spectra of hydrogen-bonded a complexes between hydrogen halides and acetylene in solid argon have been reported in an earlier paper from this laboratory.' These complexes were formed by condensation of C2H2 and HX with excess argon or by vacuum-UV photolysis of vinyl halides. All of the complexes were characterized by a strong absorption in the HX stretching region, which was shifted to lower energy than the monomeric HX fundamentals, and an intense perturbed bending mode for the C2H2submolecule in the complex. On the basis of the spectra of several chemically related complexes, the C2H2-HX complex was posulated to have a "T-shaped" structure (I) with C2"symmetry. This structure has been determined for the C2H2-HC1 complex by a recent microwave investigation? With this geometry, the molecular (1)McDonald, 5. A.; Johnson, G. L.; Keelan, B. W.; Andrews, L.J. Am. Chem. SOC.1980,102,2892.
(2)Legon, A. C.;Aldrich, P. D.; Flygare, W. H. J . Chem. Phys. 1981, 75. 625. 0022-3654/82/2086-3374$01.25/0
z
t F
I
H
-
H-CEC-H
x
I
complex has 12 vibrational modes (5A1, 1A2,4B1, and 2BJ; all except one are infrared active, but several may be very weak. In the earlier study, only the two strongest vibrational modes (corresponding to the H-X stretching motion and a perturbed acetylene vgC bending motion) were identified; the other modes were not observed because of low intensity or lack of resolution from parent absorptions. In the present study, the C2H2-HX complexes have been reinvestigated by high-sensitivity Fourier transform infrared (FTIR) spectroscopy to identify other modes of the complexes. The use of vinyl halide photolysis has been employed here to minimize the interference from uncom0 1982 American Chemical Society
FT I R Spectra of C,H,-HX
and C,HX-HX
The Journal of Physical Chemistry, Vol. 86, No. 17, 1982 3375
plexed acetylene absorptions. Experimental Section The synthesis of acetyleneHX and haloacetylene-HX complexes for infrared matrix-isolation studies has been detailed elsewhere.' Briefly, vinyl halides in argon (Ar/VX = 400/1) were condensed at 2-3 mM/h for 10-24 h on a 20 K CsI plate with argon resonance radiation3 provided by argon flowing through a 3-mm orifice quartz discharge tube powdered by a microwave diathermy unit; blank experiments were performed for each sample without argon resonance radiation. Other methods involved high-pressure mercury arc photolysis of a vinyl bromide sample and simple codeposition of argon/acetylene and argon/HX samples for 10-29 h each at 2 mM/h. Vinyl fluoride and 1-chloro-1-fluoroethylene(Peninsular Chemresearch), and vinyl chloride and acetylene (Matheson) were purified through vacuum distillation. Vinyl bromide (Aldrich Chemical Co.) was degassed by several freeze-thaw cycles and used without further purification. C2DBClwas prepared by dehydrohalogenation of 1,2-dichloroethane-d4 (Merck Sharp and Dohme) with NaOD in D20, and a deuterated vinyl fluoride sample was given to us by R. L. Kuczkowski. HF and DF were prepared by the reaction of Fz (Matheson) and H2 (Air Products) or D2 (Airco) in the stainless steel vacuum system. Fourier transform infrared (FT IR) spectra were recorded between 4000 and 400 cm-' at 1 cm-' resolution with a Nicolet 7199 FT IR by using a KBr beamsplitter and a cooled Hg-Cd-Te detector; reported frequencies are accurate to k0.3 cm-' and reproducible within these experiments to i0.2 cm-'. Far-IR spectra were recorded between 425 and 125 cm-' at 2-cm-' resolution by using a 6.25-pm mylar beamsplitter and TGS d e t e ~ t o r . ~
Results Matrix isolation experiments with a number of chemical stoichiometries will be described in turn. C$13F. The direct synthesis of the C2H2-HF complex was repeated here to examine the entire 200-4000-cm-' spectral region for weaker absorptions in a relatively thick sample. Figure 1 illustrates part of the spectral region of interest for a 29-h experiment codepositing Ar/C2H2 = 300/1 and Ar/HF = 150/1 samples at 12 K. Acetylene, hydrogen fluoride, and hydrogen fluoride species bands are labeled A, HF, and D, respectively. The 3554-cm-' band, due to the H20-HF complex! and water absorptions are labled C and W,respectively. The two strongest absorptions of the C2H2-HF complex, reported previously,l where completely absorbing in this experiment; these bands, measured with the FT IR at 3745.5 (v,) and 758.4 cm-' ($), are labeled in the figure. New absorptions of particular interest were observed at 1972.5 (v2c in the figure), 1398.9 and 1347.7 (vc), 642.7 (v:), and 426.4 cm-' (vJ; a strong 696.2-cm-' band was observed on a broad absorption at 685 cm-'. The spectrum of an acetylene matrix sample without HF revealed none of these absorptions; although weak absorption appeared near 1970 cm-l, the major 1972.5-cm-' absorption in Figure 1 can be attributed to a reaction product. A similar experiment was performed with C2D2and the product bands of interest were observed at 3744.7, 1762.4, 1087.9, 1053.5,697.8, 559.4, 531.1, and 427.0 cm-l. Several experiments were done with C,Hz and deuterium-enriched hydrogen fluoride (DF/HF = 2/1); a com(3) Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrosc. 1978, 32, 157. (4)Johnson, G. L.;Andrews, L. J. Chem. Phys. 1982, 76,2875. (5) Johnson, G.L.; Andrews, L. To be published.
WAVENUMBERS
Figure 1. Selected reglons in the FT I R spectrum of the matrix p r a pared by codeposfflng Ar/C2H2 = 30011 and Ar/HF = 15011 samples at 12 K for 29 h. Acetylene, hydrogen fluoride, and hydrogen fluoride species bands are labeled A, HF, and D C. denotes the H,O-HF complex and W identifies water bands.
posite spectrum is shown in Figure 2. The strongest product bands were observed at 3745.5 (2.1-cm-' full-width at half-maximum, fwhm),2751.8 (1.6-cm-' fwhm) (vJ, and 759.2 cm-l (vgc);weaker product bands were observed at 640.8 (vqC), 1972.5, 1397.4, 1347.7 cm-' (not shown); the band at 696.4 cm-' exhibited a DF counterpart at 539.2 cm-'. A separate experiment devoted to the far-IR region revealed a quartet at 426.2, 382.0, 323.2, and 284.4 cm-' (v,); absorbances and fwhm for these bands are A = 0.09 and 3.5 cm-', A = 0.18 and 1.4 cm-', A = 0.19 and 3.2 cm-l, and A = 0.33 and 1.4 cm-', respectively. The mid-IR experiment was repeated with Ar/C2D2 = 300/1 and Ar/(HF + DF) = 300/1 for DF/HF > 2 / 1 samples; the strongest product bands were observed at 3744.7, 2751.4, and 560.1 cm-'; weaker product bands were observed at 1762.2, 1087.9, 1052.0, 530.4, and 426.8 cm-'. The 0.2-cm-' difference betwen the v,(HF) band measurement in two different C2Dzexperiments is probably due to the 8-fold difference in band absorbance due to acid concentration. This sample was warmed to 21 K over a 5-min period, and the above product bands increased by 60% while HF and DF monomer bands decreased and their dimer absorptions increased. Several argon-resonance photolysis experiments were done with vinyl fluoride (VF) Ar/C2H3F = 400/1samples during deposition at 20 K. The major products, as reported previously,' were strong 3745.5 (v,) and 758.4 cm-' absorptions, which are shown in Figure 3; the complete spectrum is in general agreement with the observations of Guillory and A n d r e ~ salthough ,~ the present interest is limited to product absorptions common to the HF and C2H2coedeposition experiments. The frequency accuracy and resolution of the FT IR makes possible more accurate characterization of a number of other product bands. The doublets at 3358.1-3346.1,2240.3-2236.3, 1061.2-1057.0, and 585.9-582.9 cm-' are assigned to fluoroacetylene (FA) based upon agreement with the gas-phase spectrum.6 A sharp weak band at 3288.9 cm-' and a 737-cm-l shoulder (A) are due to isolated acetylene; the sharp, strong product bands a t 3280.9 (v3c) and 739.7 cm-' ( v g C )are clearly different from acetylene and they must be due to a photolysis product. Weaker product bands were observed at 1398.9 and 1347.6 (vc), 642.8 (vqC), 426.3 ( V I ) , and 695.6 cm-'. In
(vc)
(6)Hunt, G.R.;Wilson, M. K. J. Chem. Phys. 1961,34, 1301. (7)Guillory, W. A.; Andrews, G. H.J. Chem. Phys. 1975, 62, 3208, 4667.
3376 The Journal of Physical Chemistry, Vol. 86, No. 17, 1982
Andrews et al.
0 ' 0
F1
IO
34b0
37b0
31b0
28b0
25b0
81
1
500
455---7 50
WAVENUMBERS
Flgure 2. Regions of interest in the FT I R spectrum of a matrix prepared by codepositing Ar/C2H, = 30011 and Ar/(HF iDF) = 20011, DF/HF F= 211 samples at 12 K for 24 h. Mid-IR scanned wlth l-cm-' resolution; far-IR wlth 2-cm-' resolution. t
I
3800
3700
r
3300
t
-
3200
1300
I 700
600
500
900
WAVENUMBERS
M 3. selected regions In the FT I R spectrum of a matrix prepared by codeposltlng Ar/C,H,F = 40011 at 20 K with argon resonance radiation for 10 h. Acetylene, fluoroacetylene, and vlnyl fluorlde bands are labeled A, FA, and VF; C02 has been removed from the spectrum.
F
similar experiments with a Beckman IR-12 spectrophotometer, a doublet was observed at 426.5 (A = 0.14) and 382.0 cm-' (A = 0.28) in the far-IR. A similar experiment was done with a 62% deuteriumenriched vinyl fluoride sample; the spectrum was complicated owing to the isotopic mixture; however, several product absorptions are of interest here. Doublets at 2645.9-2639.4, 2059.6-2052.7, 1046.0-1041.8, and 442.0 cm-'are assigned to fluoroacetylene-d,, The major product absorptions were sharp bands at 3744.9 and 2751.5 cm-'; a sharp 2442.0-cm-I band is due to C2D, with a sharp new counterpart at 2433.6-cm-'; sharp new bands were observed at 560.2 and 545.3 cm-l, just above CzD2at 542.0 cm-', and at 426.9 cm-l. The mixed isotopic acetylene CzHD was observed at 683.5 cm-', with sharp new products at 686.0 and 707.3 cm-', at 519.3 cm-' with a new band at 521.2 cm-', and at 2586.6 cm-' with a new band at 2579.0 cm-l. C&13CZ. Several argon-resonance photolysis experiments were performed with vinyl chloride (VC) Ar/C2H3C1= 400/1 samples during condensation at 20 K. The major product absorptions are shown in Figure 4; the strongest product absorptions were 2764.4 (us) and 751.6 cm-' (usc).
Sharp doublets a t 3341.0-3326.4, 2115-2109.3, and 606.5-605.2 cm-' are assigned6 to chloroacetylene (CA). Sharp, weak 3303.2- and 3289.0-cm-' bands are due to acetylene (A), but the sharp, strong 3282.6- and 737.2-cm-' bands are due to a different photolysis product. Weaker product bands were observed at 1973.1 (uzc), 1343.9 (uC), and 629.8 cm-' (udC). In a vacuum-UV photolysis experiment with the Beckman IR-12, a new product band was observed at 240 cm-' in the far-IR. A similar experiment was done with a partially deuterated vinyl chloride sample. The major product bands, not appearing in a similar sample deposited without vacuum-UV photolysis, were 2764.0 and 2003.4 cm-' (us); a sharp, new band was observed 2435.1 cm-' below CzDzat 2442.0 cm-'; sharp product doublets appeared at 554.0-543.8 and 699.4-684.5 cm-l. Chloroacetylene-dlwas observed at 2602.3 and 1976.5 cm-'. C2H3Br. Two experiments were performed with Ar/ C2H3Br= 400/ 1mixtures. In the first study, a sample was deposited a t 15 K and then irradiated by 220-1000-nm high-pressure mercury arc light for 30 min; new bands appeared at 3282.1, 2466.5 ( A = 0.1), 750.2, and 737.4 cm-'.
FT I R Spectra of C,H,-HX
The Journal of phvsical Chemistty, Vol. 86, No. 17, 1982 3377
and C,HX-HX
vc 1°C
0
3300
'
I
--
2 a b 0 0 0 1900 WeVENUMBERS
1300 7
b
0
vc/
0
0
Flgure 4. Regions of interest in the FT IR spectrum of a matrix prepared by argon resonance photolysis of an Ar/C,H,CI = 400/1 sample during condensation at 20 K for 15 h. Acetylene, chloroacetylene, and vinyl chloride bands are labeled A, CA, and VC; COP has been removed from the spectrum.
A similar amount of the same sample was deposited concurrently with argon resonance radiation and the same product bands were observed with an 8-fold increase in absorbance along with weak 1972.8- and 1342.9-cm-' bands. Bromoacetylene absorptions were observed a t 3333.5-3321.0,2092.1-2086.6 and 615 cm-'and sharp, weak acetylene absorption was found at 3302.6-3288.8 cm-'. Photolysis of this sample for 30 min with the mercury arc slightly decreased the C2H3Brabsorptions, increased the former product absorptions by 25%, and increased the bromoacetylene bands by 100%. C&$CZ. Two argon resonance photolysis experiments were done with fluorochloroethylene; the most interesting regions in the product spectrum are shown in Figure 5. Sharp, strong product bands were observed a t 3769.3-3763.7,3347.9,3337.5,3326.2,3313.3,2816.3-2790.9, 2236.3, 2232.3, 2109.3, 2100.9, 1061.6-1056.3,627.9,603.9, 596.6, and 581.4 cm-'. Weak acetylene bands were observed a t 3303 and 737 cm-'. Other medium-intensity product bands a t 1456, 1300, 1202, 1122, and 888 cm-' cannot be identified without more information.
I us I
1
32b0
3900
3600
3800
Discussion Earlier studies's2 have established the Czu "T-shaped" structure for the acetylenehydrogen halide ?r complexes. The present infrared observations will be considered for complementary information on the spectroscopy, bonding, and structure of the complexes. Assignments, C$12-HX. The vibrational spectrum of a hydrogen-bonded complex contains bands due to the acid and base submolecules which differ from the free acid and base molecules depending on the strength and orientation of the hydrogen-bonding interaction. The free molecule spectra thus provide a reference point for the vibrations of the submolecules in the complex, which may split degenerate vibrations of the base molecule. The strongest absorptions for H-F in a hydrogen-bonded complex are the perturbed H-F stretching mode (vJ, and the librational motion of H-F in the complex (vJ, which is doubly degenerate for a linear complex. The lower frequency modes (v, and v ) derived from former translational degrees of freedom Bare weaker and more difficult to observe. The
Figure 5. Regions of interest In the FT I R spectrum of a matrix prepared by argon resonance photolysis of an Ar/C,H,FCI = 400/1 sample during condensation at 20 K for 17 h. Fluoroacetylene and chloroacetyleneabsorptions are labeled FA and CA.
( 8 ) Pimentel, G. C.; McClellan, A. L. "The Hydrogen Bond"; W. H. Freeman: San Francisco, 1960.
(9) Herzberg, G. "Infrared and Raman Spectra"; Van Nostrand Princeton, 1945.
2900
'
27b0 22kO
2050
WAVENUMBERS
absorptions for the C2H2submolecule in the complex will be related to C2H2vibrations and assigned as vic following the ith mode assignments of C2H2i t ~ e l f . ~ The absorptions given in Table I are assigned to the C2H2-HX complexes based on their formation from cocondensation of HX and C2H2molecules and photodissociation of CH2CHX molecules. Additional support comes from isotopic substitution of both acid and base submo-
Andrews et al.
3378 The Journal of Physical Chemistry, Vol. 86, No. 17, 7982
TABLE I : Absorptions ( c m - ' ) Assigned to the HX and C,H, Submolecules in C,H,-HX Complexes in Solid Argon
C,H,-HX
-
CZH,
HF
HCl
HBr
DF
2888.0 2764.4 3282.6 1973.1
2569.0 2466.5 3282.1 1972.7
2895.8 2751.8
3288.9 1973.8 1334.4 1334.4 736.9 736.9 612
3953.8 3745.5 3280.9 1972.5 1398.9 1347.7 758.4 739.7 642.7 426.2 382.0
1343.9 751.6 737.2 629.8 (240)
1342.9 750.2 737.4
HX VS
v3 V2
(v, (v,
+ v,)(xz)
+ v5)(xy)
V&Z)
v .fxv
v&i j VI(=
1
Vl(YZ)
C ,D ,-HX
lecules. The strong 3745.5-cm-' band is assigned to v, of the C2H2-HF complex based upon its position below the 3953.8-cm-' fundamental for HF in solid argonlo and its HF/DF ratio, 3745.5/2751.8 = 1.3611, which is almost the same as the acid molecule ratio 3953.8/2895.8 = 1.3654. This mode shows a slight shift to 3744.7 cm-' for the C2D2-HF complex and to 2751.4 cm-I for the C2D2-DF complex confirming that C2H2is included in the species responsible for this vibration, but C2H2is involved only to a very small degree in this normal mode. A similar very small isotopic shift was observed in us for deuterium substitution in the base submolecule in the C2H4-HF complexes." The 426.2- and 382.0-cm-l bands are assigned to the modes, one motion in the x z plane (parallel to the C d ! bond) and the other in the yz plane (perpendicular to the C=C bond). The slightly different HF/DF ratios 426.21323.2 = 1.3187 and 382.01284.4 = 1.3432 are reasonable for this vibration. It is not possible to definitively assign each VI component to a specific one of the above planes of motion, although a preference for assignments can be given. The y component intensities and bandwidths differ by factors of 2 and 0.5 such that the integrated intensities are similar. Since the v$(xz) mode of the C2H2 submolecule in the complex exhibits a large displacement (21.5 cm-l), which might in part be due to repulsive interaction between the acetylenic hydrogens and the acid hydrogen, the presence of acetylenic hydrogens might sharpen the v I ( x z ) potential and enhance the anisotropy of the cylindrical 7r electron system. Two other observations support this rationale: first, the analogous C2H4-m complex" without axial protons exhibits v1 modes at 423.8 and 396.1 cm-l, a splitting 6.8% of the average value, substantially less than the 13.7% splitting for vl of CzH,-HF; and second, the analogous propyne and butyne complexes exhibit y modes at 490.3,432.6 and 552.3,489.3 cm-', respectively,12splittings of 12.5 and 12.1%. Hence replacing the acetylenic proton by a methyl group decreases the relative splitting in y. It is therefore suggested that the higher frequency y component is due to libration in the x z plane and the lower frequency vl component is due to libration in the y z plane. The additional band at 696.2 cm-' might be considered for possible assignment to the first overtone of vl. The intensity of the 696.2-cm-l band relative to the 426.4-cm-' fundamental is approximately constant at 113 except for the most concentrated HF experiment (Figure 1) where the ratio is approximately 1/2. The HF/DF ratio 696.2/539.2 = 1.291 is reasonable for an overtone band (10) Mason, M. G.; Von Hone, W. G.; Robinson, D. W. J. Chem. Phys. 1971,54, 3491. (11) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Chem. Phys. 1982, 76. . -, 5767. - .- .. (12) Andrews, L.; Johnson, G. L. J. Phys. Chem. Following article.
1972.5 1397.4 1347.7 759.2 640.8 323.2 284.4
C,D,
2442.0 1762.4 1043.3 1043.3 542.4 542.4 512
HF
DF
DCl
3953.8 3744.7
2895.8 2751.4 2433.6 1762.2 1087.9 1052.0 560.2 545.3 530.4
2089.0 2003.4 2435.1
1762.4 1087.9 1053.5 559.2 531.1 427.0
554.0 543.8
which is more anharmonic than the fundamental. I t is puzzling, though, for only one possible overtone band to be observed for a split fundamental. A more likely explanation for the 696.2-cm-' band is a v1 type mode in C,H,-(HF), complex. The strong 758.4-cm-' band is assigned to vgC in the C2H2-HF complex based upon its position and C2H2/C2Dz ratio 758.4/559.2 = 1.3562 which is near the acetylene ratio 736.9/542.4 = 1.3586. The small DF shift in vgCfor C2H2-DF to 759.2 cm-l and C2D2-DF to 560.2 cm-' again verifies a small but definite interaction between the submolecules in the complex. A similar small DF shift has been observed for vTC of the C2H4complex.ll Now, the HF ligand splits the degneracy in v5 of C2H2and it is expected that the in-plane v ~ ' ( x z mode ) will sustain a greater perturbation that the out-of-plane v$(xy) mode. On this basis, the strong 739.7-cm-' band, resolved from acetylene only in VF photolysis studies and distinctly different from acetylene produced in lower yield at 736.9 cm-l, is assigned to v5C(xy)(B2),and the strong 758.4-cm-' band is assigned to ~5'(~z)(Ai).A similar splitting was found for usc of C2D,-DF at 560.2 and 545.3 cm-l, above CzD2at 542.4 cm-l, and vgCof C,HD-DF at 707.3 and 686.0 cm-l, above C2HD at 683.5 cm-'. The sharp, strong 3280.9-cm-' band is clearly different from the weak A band at 3288.9 cm-' in the VF photolysis experiments. The strong band is assigned to v3c in the C,H,-HF complex, which exhibits a small Av3 of -8.0 cm-'. The matrix photolysis experiment produced and trapped a substantially higher yield of complexed products than isolated products. An interesting aspect of the C2Hz-HF complex is to determine if the inactive Z,+(vl and v2) and II,(v4) modes for acetylene can be observed in the complex; both vlC and vZcare IR active in Czusymmetry, ~ ~ ~ (isxactive z ) (Bl), but ~ ~ ~ ( isx the y ) unallowed A, mode. The sharp, weak 1972.5-cm-l band is assigned to v2c based on its proximity to the 1973.8-cm-' gas-phase value? the C2D2shift to 1762.4 cm-', which is in exact agreement with the gas-phase value, and the slight H-X shift in this mode (Table I). These observations show that skeletal stretching modes (v3 and vl) are affected very little by the hydrogen-bonding interaction. The vlC mode was not observed; it may be obscured by v1 of FA. The stronger 642.7-cm-l band, assigned to vqC(xz),was produced with both synthetic methods, this mode is shifted 31 cm-I above the 612-cm-' gas-phase value by the HF perturbation in the complex whereas Y ~ ' ( X Z ) was blue shifted only 21.5 cm-'. The v4'(xz) band for C2Dz-HF was observed at 531.1 cm-l, above a 512-cm-' estimated v4 value for CzD2in the gas phase? The v4 + v5 combination band is infrared active for the C2Hzmolecule. In the C2H2-HF complex, this combination level is split into two active modes, ~ ~ ~ (+x v5'(xy) y ) and
The Journal of Physical Chemistry, Vol. 86, No. 17, 1982 3379
FT I R Spectra of C2H2-HX and C,HX-HX
+
v ~ ' ( x z ) v;(xz), both B1, and the inactive orthogonal cross combinations, both AP The 1347.7-cm-' band is assigned to v4'(xy) v&y) based on agreement with the sum 612 740 = 1352 cm-' (and the assumption that v4'(xy) will be essentially unchanged from v4 in the gas phase); the complex is less anharmonic than the molecule where v4 + v5 = 612 729 = 1341 cm-l (1328 cm-' observed in gas phase). The 1398.9-cm-' band is assigned to v ~ ' ( x z ) + v ~ ' ( x z )owing to agreement with the sum 642.7 758.4 = 1401.1, again displaying little anharmonicity. The slight changes in Y ~ ' ( X Z ) and Y ~ ' ( X Z ) for the C2H2-DF complex, to 640.8 and 759.2 cm-l, are manifested by a small change in the combination band at 1397.4 cm-', 1.5 cm-' below the 1398.9-cm-' value for C2H2-HF. Similar agreement is found in the C2D2-HF complex. The 1053.5-cm-' band is assigned to the ( x y ) combination based on the sum 512 + 545 = 1057 cm-', and the 1087.9-cm-' absorption is assigned to the ( x z ) combination owing to the sum 531.1 + 559.3 = 1090.4 cm-'. The observation of the two allowed combination bands of the complex in near agreement with the sum of complex fundamentals confirms the bending mode analysis for the C2H2submolecule in the C2H2-HF complex presented here. The strong 2764.4-cm-' band is assigned to v, in the C2H2-HC1 complex based upon its position below the 2888-cm-' fundamental for HC1 in solid argon and the HCl/DCl ratio, 2764.0/2003.4 = 1.380, which is near the diatomic molecule ratio 2888/2089 = 1.382. The 240-cm-l band in the vinyl chloride photolysis experiment could be due to vI, but without DC1 data, a definitive assignment cannot be made. The single 240-cm-' band is broader (8 cm-' fwhm) than the split vI bands for the HF complex; however, owing to the weaker hydrogen bond in the HC1 complex as compared to the H F complex, anisotropy in the librational potential may be insufficient to resolve the splitting in v1 for C2H2-HC1. This possible explanation is consistent with the observation of very little difference in average angular displacements of HC1 in the xy and x z planes from the microwave measuremenh2 The strong 751.6- and 737.2-cm-' bands are assigned to the two vgC components for the C2H2submolecules split by the HC1 ligand; likewise, the 554.0-543.8-cm-' doublet is due to the v5' components of C2D2split by DC1. These displacements above v5 for C2H2(and C2D2)in the HC1 (and DC1) complexes are less than for the stronger hydrogen-bonding HF ligand. This trend is also shown in Table I for several other submolecule modes. Note, in particular, that the small displacement in v3' for the HF (and DF) complexes is less still for the HC1 (and DC1) complexes. The strong 2466.5-cm-' band is assigned to v, in the C2H2-HBr complex based upon its position below the 2569-cm-' HBr fundamental in solid argon. The strong 750.2- and 737.4-cm-' bands are due to the usccomponents split by the HBr ligand. The displacements above v5 are the smallest for the HBr ligand, which forms the weakest hydrogen bond in the series studied here. Structure, C2H2-HX. More vibrational spectroscopic evidence for the C2"structure of the complex in I is provided by the bending modes of the C2H2submolecule and the librational modes of the HF submolecule in the complex. First, the splitting of v1 confirms that the complex is nonlinear, and, as discussed above, the magnitude of this splitting is appropriate for the anisotropic potential governing the libration of HF against the a system of C2H2. Second, the splitting of v5' also confirms that the complex is nonlinear. Third, the observation of usc and vqC displaced 21 and 31 cm-' above us and v4, respectively, shows that
+
+
+
+
TABLE If: Absorptions (cm-I) Assigned to the HX and C,HX Submolecules in C,HX Complexes in Solid Argon C,HF-HX C,HF HX VS VI VZ v3
V,(XZ)
v,(xy)
3358.1 2240.3 1057.0 585.9 582.9
HF(n)
HCl(n)
3953.8 3776.0
2888.0 2790.9 3337.5 2232.2 1061.6 596.6 581.4
1064.6 612 582.5
C,DF
2895.8 2775.7 2645.9 2059.6 1046.0 442.0 442.0
C,HCl-HX C2HCl
H F ( n ) HCl(n)
C,DC1
3326.4 2109.3 606.5 605.2
3953.8 3763.7 3313.3 2100.9 627.9 603.9
2602.3 1976.5
HX US VI v2
v,(xz)
v,(xy)
2888 2784 3316 618 606
C,DFDF(n)
459 443 C,DC1DCl(n)
2003.4 2596 478 469
the bending modes of the C2H2submolecule are altered slightly in the complex, which must maintain some of the original symmetry of the C2H2molecule. The C%structure for the complex satisfies the above conditions imposed by the infrared spectrum. C a X - H X . The CH2CFClphotolysis experiments were done to determine the relative photoelimination of HF or HC1 from the same precursor and to prepare the C2HClHF and C2HF-HC1 complexes. The strong absorption peak at 3763.7 cm-' is clearly due to the v5 mode of HF in a a complex with chloroacetylene (CA). This absorption is intermediate between the 3776.0- and 3745.5-cm-' values for HF in a complexes with FA and A, respectively.' The decrease in Avs is due to the stronger inductive effect of F as compared to C1 on the 7r system which reduced its Lewis basicity. The absorption peak at 2790.9 cm-' is likewise due to the v5 mode of HC1 in a a complex with FA. This vibration is higher than the 2784- and 2764-cm-' values for HC1 in a complexes with CA and A, respectively, which is also explained by the inductive effect of the haloacetylene substituent. The C-H stretching region provides further product identification; sharp, weak bands at 3347.4 and 3326.2 cm-l are in agreement with one component of the matrix split band for FA and CA, respectively, and the sharp, strong 3337.5- and 3313.3-cm-' bands are assigned to up modes of the C2HF-HC1 and C2HC1-HF a complexes. The displacements for the vlC modes are relatively small for the haloacetylene submolecules in the complexes and are comparable to displacements in vlC for the acetylene complexes. The c--=C stretching region again reveals weaker product bands, 2236.3 and 2109.3 cm-', due to the isolated FA and CA molecules and stronger 2232.3and 2100.9-cm-' bands due to vZcmodes for the a complexes. The bending mode region exhibits four product bands; the stronger 603.9- and 581.4-cm-' bands obscure weaker 605- and 584-cm-' absorptions due to isolated C and FA, respectively. The 627.9- and 603.9-cm-' bands are assigned to the two components of vqCfor the C2HC1-HF a complex; the higher band corresponds to bending in the x z plane (Av4 = 23 cm-') and the lower band is due to bending in the xy plane (Av4 = 2 cm-'). The 596.6- and 581.4-cm-l bands are assigned to two components of vqCfor the CzHCl a complex; the higher band involves bending in the x z plane (Av4 = 13 cm-') and the lower band is due to bending in the xy plane (Av4 = -2 cm-'1. The large A v ~ ( x zvalues ) correspond to 21- and 15-cm-' values for the C2H2-HF and C2H2-HC1complexes; the small negative
3380
J. Phys. Chem. 1982, 86, 3380-3385
Av4(xy)values correspond to small positive values in the acetylene complexes. The matrix photolysis products of CH2CFClare clearly dominated by ?r complexes rather than isolated haloacetylenes; HF photoelimination is favored over HCl by a factor of 1.6 f 0.1 based upon the relative yields of the ulC and v2cabsorptions for the C2HC1-HF and C,HF-HCl complexes and the u1 and v2 bands for the isolated CzHCl and CzHF molecules. The major product in previous dichloroethylene photolysis experiments' was identified as the C2HC1-HCl x complex from v, at 2784 cm-l; in addition, the spectra reveal the ulC mode at 3316 cm-' and split vqCcomponents at 618 and 606 cm-', which are compared with the present observations for C,HX-HX complexes in Table 11. Previous assignments' to the C2HF-HF and C2DF-DF x complexes are also given to complete the comparison.
Conclusions High-sensitivity, high-resolution FT IR matrix isolation studies have observed several new vibrational modes for C2H2-HX and C,HX-HX complexes produced by HX
addition to acetylene and by vacuum-UV photolysis of suitable vinyl halides and dihaloethylenes. The complexes were characterized by an intense, strongly perturbed HX stretching fundamental v, and by slightly perturbed stretching and bending fundamentals v2c, usc, vqC,and usc of the acetylene submolecule. A comparison of the relatively small shifts for the stretching and relatively larger displacements for the in-plane bending fundamentals in the acetylene moieties and the loss of degeneracy in all observed bending motions support the identification of a hydrogen bonded 7~ complex C2H2-HX with C , symmetry. The split components of the HF libration ul in the CzH2-HF complex show substantial anisotropy for the hydrogen-bonding potential in the x complex. Evidence is presented for the ?r complexes C2HF-HCl and C,HCl-HF produced upon matrix photolysis of C2H2FC1. Acknowledgment. The authors gratefully acknowledge financial support from the National Science Foundation under Grant CHE79-10966 and the gift of the deuteriumenriched vinyl fluoride sample from Drs. K. Hillig and R. L. Kuczkowski.
Fourier Transform Infrared Spectra of Substituted Alkyne-Hydrogen Fluoride Complexes in Solid Argon Lester Andrews" and Gary L. Johnson Department of Chemistry. Universiv of Virginia, CharloiTesville. Virginia 2290 1 (Receive& March 22, 1982)
Hydrogen-bonded x complexes with alkynes and HF have been prepared by reagent cocondensation and by vacuum-UV photolysis of CH2CFCH3for matrix Fourier transform infrared (FT IR) spectroscopic study. The v, and vI HF submolecule modes show an increasing hydrogen-bond strength with increasing methyl substitution; splitting in the v1 modes arises from anisotropy in the hydrogen-bondinginteraction with a A system. Alkyne submolecule deformation modes are increased and skeletal stretching modes are decreased in the complex; splitting in the deformation modes is characteristic of a nonlinear complex to the x system. Evidence is presented for both u and A complexes with CzHF and HF.
Introduction A considerable amount of research has been performed recently on hydrogen-bonded T complexes of acetylene or ethylene and hydrogen halides.14 Infrared matrix-isolation'r3 and microwave4 spectra have been interpreted to indicate a "T-shaped" structure (Scheme I) for the acetylene x complexes. Similar studies of substituted alkenes have shown that methyl substitution gives a stronger hydrogen bond, as determined by u, and ul modes of the complex, and that halogen substitution gives a weaker hydrogen bond in a u complex involving the halogen.6 The (1) McDonald, S. A.; Johnson, G. L.; Keelan, B. W.; Andrews, L. J. Am. Chem. SOC.1980,102, 2892. (2) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Chem. Phys. 1982, 76, 5767. (3) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Phys. Chem. Preceding article in this issue. (4) Legon, A. C.; Aldrich, P. D.; Flygare, W. H. J. Chem. Phys. 1981, 75, 625. (5) Aldrich, P. D.; Legon, A. C.; Flygare, W. H. J. Chem. Phys. 1981, 75, 2126. (6) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Am. Chem. SOC. accepted for publication.
Scheme I Z
X
.
H-CEC-H
-
x
y mode of HF in the complex is split, indicating anisotropy in the hydrogen-bondinginteraction with the x s y ~ t e m . ~ ? ~ . ~ Similar complexes of HF and methylacetylenes and fluoroacetylene have been prepared to examine hydrogen-bond strength and structure in the complexes.
Experimental Section The vacuum and cryogenic techniques used in the present experiments have been described p r e v i o ~ s l y . ~ ~ ~ ~ ~ ~ Briefly, three methods were used here t o produce alk(7) Kelsall, B. J.; Andrews, L. J. Phys. Chem. 1981, 85, 1288. (8) Andrews, L.; Johnson, G. L. J. Chem. Phys. 1982, 76, 2875.
0022-3654/82/2086-3380$01.25/00 1982 American Chemical Society
