J . Phys. Chem. 1984, 88, 425-432
425
Fourier-Transform Infrared Spectra of (HF), Species in Solid Argon Lester Andrews* and Gary L. Johnson Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: April 6, 1983)
Dilute mixtures of HF in argon condensed at 12 A 1 K produced strong, sharp absorptions in the 4000-3600-cm-' region, broader bands between 3500 and 3100 cm-', and new absorptions in the 1250-1 50-cm-l region, which exhibited substantial HF concentration dependences. These new bands changed by different fractions on temperature cycling the sample, and reached maximum absorbances at different times with the sample at 16-28 K, which provides strong evidence for stepwise association of HF to give a number of different (HF), species. Isotopic experiments with enriched DF yielded (DF), counterparts and satellite absorptions for mixed (HF),(DF), species. Two H-F stretching fundamentals for (HF), were found at 3896 and 3825 cm-I, and the strongest absorption of an open trimer appeared at 3702 cm-I. Four different groups of bands between 3427 and 31 18 cm-' are assigned to cyclic-(HF), species ( n = 3, 4, 5 , 6). The controlled diffusion of HF in solid argon provides a mechanism for the stepwise association of HF and isolation of (HF), species (2 5 n 5 6) for spectroscopic study.
Hydrogen fluoride is a simple molecule that undergoes extensive self-association in the vapor phase. Infrared,'" electron diff r a ~ t i o n and , ~ mass spectroscopic studied have shown that the molecular composition of hydrogen fluoride vapor is complex due to the formation of aggregates by intermolecular hydrogen bonding. Owing to the chemical and biological importance of hydrogen bonding, the hydrogen fluoride system is a key subject for examination of the hydrogen-bonding phenomenon. Hydrogen fluoride dimer is a particularly simple hydrogen-bonded species which is stable as a free molecule. The dimer was first characterized by infrared absorptions at 3896 and 3857 cm-' in the gas phase,'s3 and by radiofrequency and microwave spectra which indicated a semirigid, bent structure.6 There have followed higher resolution infrared spectra of the and finally a resolved vibrational-rotation spectrum9 appeared while this manuscript was in preparation; this latter work9 agrees with the microwave results on the molecular structure and the early gas-phase work on the vibrational assignments. Numerous t h e ~ r e t i c a l ' ~ and '~ potential s u r f a ~ e ' ~calculations ,'~ on (HF), have been done in the last decade, and the dimer has been observed in vibrational predissociation studies16 which offered different vibrational assignments from the original infrared work. The higher (HF), species have not been well characterized, and there is disagreement between gas-phase infrared and predissociation ~ p e c t r a . ' ~Recent ~ ~ ' ~ laser absorption measurements of H F vapor have been interpreted in terms of the earlier tetramerhexamerI7 and tetramer through dodecamer models.Is Infrared spectra of these species as a function of n will provide structural and bonding information, and the matrix isolation technique is particularly well suited for such a study. Two earlier matrix infrared studies were concerned with the monomer region of the s p e c t r ~ m . ' Fourier-transform ~~~~ infrared (FTIR) spectroscopy (1) Smith, D. F. J . Chem. Phys. 1958, 28, 1040.
(2) Kuipers, G. A. J. Mol. Spectrosc. 1958, 2, 75. (3) Smith, D. F. J . Mol. Spectrosc. 1959, 3, 473. (4) Van Huong, P.; Couzi, M. J . Chim. Phys. 1969,66, 1309. (5) Janzen, J.; Bartell, L. J . Chem. Phys. 1969, 50, 3611. (6) Dyke, T. R.; Howard, B. J.; Klemperer, W. J . Chem. Phys. 1972,56, 2442. (7) Herget, W. F.; Gailar, N. M.; Lovell, R. J.; A. H . J . Opt. SOC.Am. 1960, 50, 1264. (8) Himes, J. L.; Wiggins, T. A. J . Mol. Spectrosc. 1971, 40, 418. (9) Pine, A. S.; Lafferty, W. J . J . Chem. Phys. 1983, 78, 2154. (IO) Dill, J . D.; Allen L. C.; Topp, W. C.; Pople, J. A. J . Am. Chem. SOC. 1975, 97, 7220 and references therein. (11) Curtiss, L. A,; Pople, J. A. J . Mol. Spectrosc. 1976, 61, 1. (12) Swepston, P. N.; Colby, S.;Sellers, H . L.; Schafer, L. Chem. Phys. Lett. 1980, 72, 364. (13) Umeyama, H.; Morokuma, K. J . Am. Chem. SOC.1977, 99, 1316. (14) Yarkony, D. R.; O'Keil, S. V.; Schaeffer, H . F., 111; Baskin, C. P.; Bender, C. F. J . Chem. Phys. 1974, 60, 855. (15) Barton, A. E.; Howard, B. J. Faraday Discuss. Chem. SOC.1982, 73, 45. (16) Lisy, J. M.; Tramer, A.; Vernon, M. F.; Lee, Y . T. J . Chem. Phys. 1981, 75, 4733. (17) Hinchen, J. J.; Hobbs, R. H . J . Opt. SOC.Am. 1979, 69, 1546. (18) Reddington, R. L. J . Phys. Chem. 1982, 86, 561.
0022-3654/84/2088-0425$01.50/0
provides improved signal-to-noise in the 3000-4000-cm-~ region where matrix samples transmit poorly, and closed-cycle helium refrigeration gives the continuously variable substrate temperature needed for controlled diffusion to produce H F association products in a stepwise fashion. A preliminary account of the H-F stretching region in the hydrogen fluoride argon matrix system has been reported.,' Here follows a detailed FTIR spectroscopic study of the H F and DF systems in solid argon.
Experimental Section The vacuum and cryogenic apparatus and FTIR spectroscopic techniques have been described in earlier reports; 22,23 1000 scans were obtained at 1 .O-cm-' resolution from 400 to 4000 cm-' and at 2.0 cm-' from 125 to 425 cm-' with an accuracy of Et0.3 and f0.5 cm-', respectively. Hydrogen and deuterium fluoride were synthesized by reacting the elements (Matheson) at low pressure in a stainless steel vacuum system exposed only to these elements and argon for more than 1 year. Several complementary experiments were done with hydrogen fluoride (Matheson) which was condensed at 77 K and evacuated to remove volatile impurities. Hydrogen fluoride diluted with argon was condensed on an 12 f 1 K CsI window at 2-3 mmol/h for 16-20-h periods, and FTIR spectra were recorded. Selected samples were warmed to a specific temperature for a given time determined to give diffusion and association of H F from previous studies with added base molecules; 22-27 the samples were recooled to 12 K and more spectra were recorded. Changes in band absorbances upon sample warming are reported as absorbance ratios (after/before). Other samples were warmed to a specific temperature and held while sets of 100 scans at 2.0-cm-' resolution were recorded in 1.5 min at 10-min intervals until no further changes in the spectra could be detected; this process was repeated with the sample maintained at successively higher temperatures. Results Matrix isolation experiments will be reported for H F and DF H F mixtures in solid argon, and the results of diffusion studies will be emphasized. HF in Argon. Experiments were done with Ar/HF = 100/1, 200/ 1, 300/ 1,400/ 1, and 600/ 1 concentrations, and spectra from
+
(19) Bowers, M. T.; Kerley, G. I.; Flygare, W. H. J . Chem. Phys. 1966, 45, 3399.
(20) Mason, M. G.; Von Holle, W. G.; Robinson, D. W. J . Chem. Phys. 1971, 54, 3491. (21) Andrews, L.; Johnson, G. L. Chem. Phys. Lett. 1983, 96, 133. (22) Andrews, L.; Johnson, G. L. J . Chem. Phvs 1982, 76, 2875. (23) Johnson, G. L.; Andrews, L. J . Am. Chem. SOC.1982, 104, 3043. (24) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Chem. Phys. 1982, 76, 5767. (25) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Phys. Chem. 1982, 86, 3374. (26) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1982, 86, 3380. (27) Johnson, G. L.; Andrews, L.J . Am. Chem. SOC.1983, 105, 163.
0 1984 American Chemical Society
426
-
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Andrews and Johnson
\ /
1
WC
0 f
90$0
3EbO
3600
3900
3200
30b0 l r b 0
I200
I000
ab0
600
900
WFlVENUMBERS
Figure 1. (a) lnfrared spectra in the 4000-3000- and 1400-400-~m-~ regions for 56 mmol of Ar/HF = 400/1 sample deposited for 20 h at 12 f 1 K. (b) Spectrum after warming to 21 f 1 K for 10 rnin and recooling to 12 f 1 K. (c) Spectrum after warming to 24 f 1 K for 10 min and recooling to 12 1 K .
*
the 400/1 experiment are shown in Figure 1. The spectrum in Figure l a is dominated by the strong HF monomer band, and sharp weaker bands labeled Q, N, D, and T, which are listed in Table I; in addition sharp waterz8 (W) and H20--HF complex29 (Wc) bands were observed. Warming the sample from 12 to 21 K for 10 min and recooling to 12 K caused pronounced changes in the spectrum, as shown in Figure l b and summarized in Table I. The N, D, and T bands at 3881.5, 3825.5, and 3702.0 cm-', respectively, exhibited ratios of 1.8, 1.9, and 7.0 (after/before) sample warming. A number of new bands appeared including sharp bands labeled 0 at 3717.5 and 3618.0 cm-l, and broader bands between 3427 and 31 18 cm-' and between 1250 and 680 cm-', which are listed in Table I. A similar thermal cycle to 24 K produced further changes; increases and decreases in band absorbances are given in the table. The sharp N, D, and T bands maintained full width at half-maximum of C2 cm-' in all of the spectra. The remaining sample was diluted to 600/ 1, the above study was repeated, and similar but weaker absorptions were observed. The N band absorbance was reduced to 80%, the D band to 50%, and the T band to 30% of their absorbances in the 400/ 1 experiment. Sample warming to 18 K for 10 min produced similar but more marked changes than in the 400/ 1 experiment; the N and D bands exhibited absorbance ratios of 1.7, the T band had a ratio 3.2, and the 0 bands exhibited ratios of 5-7 (after/before sample warming). Further warming to 21 K for 5 rnin further increased the N and D bands to 1.3, the T band to 2.2, and the 0 bands to 3 times their absorbances after the 18 K cycle. A brief experiment with 8 mmol of 100/1 sample produced comparable N and D band absorbances, but the T band was 2.5 times stronger and the 0 bands were 5 times stronger than the Figure la spectrum, and similar weak bands were observed in the 3200-3400-cm-' region. Longer experiments with 15 mmol of 100/1 sample produced a substantially stronger 3825.5-cm-' band ( A = 1.0) and a weak 3896-cm-I band ( A = 0.1) with a 3902-cm-' shoulder. Warming one of these samples to 24 K and recording (28) Reddington, R. L.; Milligan, D. E. J . Chem. Phys. 1962, 37, 2162. (29) Andrews, L.; Johnson, G . L. J . Chem. Phys. 1983, 79, 3670.
the spectrum of the warm sample increased the weak 3996- and 3877-cm-I bands, slightly decreased the 3896-, 3825.5, 3702.0-, 560-, 497-, 446-, and 401-cm-' bands, and markedly increased the broader absorptions between 3427 and 31 18 cm-l and between 1250 and 680 cm-l. Far-infrared spectra were recorded for several samples. The spectrum in Figure 2a shows product bands at 401,262, and 190 cm-I; the bands increased to new absorbances 1 . 5 4 , and 1.7 times greater after warming to 28 K for 10 rnin and recooling to 12 K. The 262-cm-' band was resolved into a 263.2, 260.7 cm-' doublet and the 401-cm-l band split into 401- and 396-cm-I components after sample warming. Several complementary experiments were done with H F (Matheson) and a Beckman IR-12 grating instrument in the 200-4000-cm-' range. The 3881- and 262-cm-' N bands were very weak in these spectra, and the 401-cm-I band tracked with the 3826-cm-' band and the 446-cm-' absorption followed the 3702-cm-' absorption in relative intensity in this series of experiments. Timed Diffusion Studies. Two experiments were done with 300/1 samples to record spectra at elevated temperatures as a function of time at 10-min intervals, and representative spectra from one of these are displayed in Figure 3 to show changes in band absorbances with increasing diffusion time. The sample was deposited at 11 K and the spectrum shown in Figure 3 was recorded. The sample was held at 16 i 1 K and more spectra were recorded; the spectrum after 20 min (no. 2) is shown in Figure 3b, and successive spectra nos. 3-5 recorded after 20-min intervals show changes in band absorbances. The sample was warmed to 20, 24, and 28 K, and spectra were recorded at 10-min intervals until no further changes could be discerned; selected spectra are shown in Figure 3c-e. The spectra show different growth and decay patterns for the product bands, which are summarized in Table I, by giving the spectrum number where that band reached maximum absorbance. DF and HF in Argon. Several experiments were performed with deuterium fluoride after 3 months of exchange in the vacuum system during other studies with DF; the DF/HF concentration ratio can be estimated from the induced DF/HF Q-branch ab-
FTIR Spectra of (HF), Species in Solid Argon
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 427
TABLE I: Infrared Absorptions (em-') Observed following Deposition of Ar/HF = 400/1 Sample at 1 2 K, Their Behavior o n Sample Warming, and Their D F Counterparts diffusion behaviora HF 21 24 absorptions K K 3996 i 3962.7 d 3953.8 d 3919.5 d i 3881.5 3896 i 3877 i 3858 i 3825.5 i 3816.0 i 3814 d 3787.7 a 3774.6 i 3756.5 d 3717.5 a 3711.2 i 3702.0 i 3690 i 3669 i 3618 a i 3554.7 3530 i 3427 a 3404 a ii 3298 a 3267 3244 a a 3186 3118 a 1250, 1240 a 86 5 ii a 850 a 730 694 :I 682 ii 560.6 i 497 i 446.3 i 401, 396d 261.9 190
d d d d d d d d d d i i d i d d d d i d d i d
i i i i i i i i i i i d d d i I I
mas 1 1 1 4
DF absorptions 2913 2895.8 2895.8 2876.7 2846.2 2858
7 9 3 9 7 1 13 5 9 3 6 12 7 4 9 9 12 12 14 16 16 (16)' 14 14 13 11 12 8 8 8
2803.6 2798.7 2794 2776.5
identificationb
V
HF, R(1) HF, R(0) HF, nonrotating HT, Q (induced) N = N --HI: D= H F , P(1) T = I~uPzs-(HF), D = (HF), T=trans-(HF), (site of D) M=N,--(HF),
N
W 2782.4 2730.4
W = H,O (0= opefl-(HF),)
W 2717.4 2710 2677 2674 2621.1 2536 2520 2436 2427 2421 2382 920 635 625 508 498 451.9 347.1 3 10, 307e 21 3.9 162
T=trans-(HF), (sitc of T)
w
(0 = open-(HF),) We = H,O--HI: W, = (H,O), cyclic (HF), cyclic (HF), cyclic (HF)4 cyclic (HI:)4 cyclic (HF)d cyclic (HF), cyclic (HF), cyclic, n = 5 and 6 lic, n = 5 and 6 cyclic, n = 5 and 6 cyclic, n = 5 and 6 lic, n = 4 lic, n = 4 T = tvans-(HF),
T = trans-(HF),, T = trans-(HI;), D = (HF),
$25
Diffusion behavior summarized: d = d e c r c x e d ; i = increased; ii = appeared. Scan number listed where absorption attaincd maximum absorbance in Figure 3. Parentheses denote tentative assignments. Data not available from Figure 3 but growth in Timcd samplc warming not done in Figure I C suggests 16. far-IR experiments. e The 307, 3 10 cni-' doublet incrcascd on sample warming a t the cxpcnse of the original 307. 300 c n r l doublcr.
'
sorbance ratio or the DF/HF absorbance ratio for products involving one DF or H F molecule multiplied by the HF/DF eigenvalue ratio 3954/2896. Detailed spectra are shown in Figures 4-6 for different spectral regions. Spectra from the 300/1 experiment in Figure 4a with about 75% DF show DF counterparts for the major sharp bands; sample warming to 26 K for 10 min caused marked changes in the spectrum of Figure 4b. The DF counterparts listed in Table I were determined on the basis of HF/DF ratios (which varied between 1.3645 and 1.3562 for the sharp bands) and behavior on sample warming. In the H F region the 3825.5-cm-' D and the 3702.0-cm-l T bands showed no shift or splitting, but in the DF region the 2803.6-cm-l D band exhibited a strong satellite at 2808.0 cm-l and the T band at 2717.4 cm-' exhibited a 2719.7-cm-' splitting. Sample warming to 26 K doubled the D bands, increased the T bands 5-fold and the 0 bands 20-fold, and produced other new satellite bands in the spectrum, which are listed in Table 11. Weaker T bands were identified at 3816.0 and 2798.7 cm-'; the sharp band at 3671.2 cm-' is probably due to a mixed T species. The 0 bands also exhibited
355
?.is
225
iis
1'25
df3VENUMBERS
Figure 2. Infrared spectra in the 425-125-em-' range for hydrogen fluoride samples deposited at 12 1 K (a) A r / H F = 300/1, (b) Ar/ ( D F H F ) = 200/1, approximately 75% D F enrichment.
*
+
TABLE 11: Mixed Isotopic Satellite Bands (cm-') for (HF),(DF),, Species in Solid Argon' I?
2 3 4 4 3
N = N,- -HF (D= (HI;),)
a
3i5
(HWn 3825Sb 3702.0 3717.5 3618 3427 3404
4 5 6 5,6 4 3 3 2 2
3298 3267 3 244 3186 3118 850 682 560 446.3 401 190
(H Ix(D F), e 2808.0b 3671.2, 2719.7 2742, 2737 2698 3478, 3 4 2 7 , 3 4 1 2 , 3287, 2574 3472, 3433, 3404, 3282, 2560 3350,2479 3300 3290,c 2466 3193, 2388 3 124,3070 830 674.518 55 I , 5 4 0 , 4 7 4 , 4 6 6 4 0 4 , 3 9 9 , 363 322 187
(DU, 2803.6& 2717.4 2730.4 26 74 2536 2520 2436 2427 2421 2382 d 6 25 498 452 347.1 307 162
a The miscd isotopic satellite b m d absorbanccs, rclativc t o (Dl,'In absorbanccs, dcpcndcd upon the DI: cnrichincnt. In I,igurc 4u the 3825-, 280%. and 2803-cnir' bands wcrc 0.07, 0.18, and 0.25 absorbance units, respectively ; tcmpcrnturc cycling to 26 K as s h o a n in Figure 4 b increased t h c absorbances to 0.1 8 , 0.36 and 0.45, respectively. The broad 3290-c1n-' band (907i DI,') w a s observed a t 3280 c n - ' with 60% DF. Not obycrvcd d u e to CO, f;ispliasc band. e .Y + J ' = n.
weaker satellites. Separate 100/1 and 200/1 experiments with 90% DF gave an increased yield of the T and 0 bands relative to the D bands and decreased intensity of the 2808.0- and 2719.7-cm-' satellite bands relative to the stronger 2803.6- and 271 7.4-cm-' absorptions. Figure 5 shows the 3600-3000- and 2650-2370-~m-~regions of the spectrum from a 200/ 1 sample of 90% enriched DF after a series of temperature cycles to 14, 20, and 25 K. The latter
428
Andrews and Johnson
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Figure 3. Infrared spectra in the 4000-3000- and 1000-400-cm-' regions for A r / H F = 300/1 sample deposited for 16 h a t 11 i 1 K: (a) Spectrum no. 1 recorded after 11 K deposition. (b) Spectra recorded at 16 K no. 2 after 20 min, no. 3 after 40 min, no. 4 after 60 min, no. 5 after 80 min. (c) Spectra recorded at 20 K; no. 6 after 20 min, no. 7 after 40 min, no. 8 after 70 min. (d) Spectra recorded at 24 K, no. 9 after 20 min, no. 10 after 40 min, no. 11 after 60 min. (e) Spectra recorded at 28 K; no. 12 after 20 min, no. 13 after 40 rnin, no. 14 after 70 min, no. 15 after 100 min, no. 16 after 130 min. The N, D, and T lables note the scans where these bands reached maximum absorbance. u
-
0
-
I
2: 1
; I -B
r
3dOO
0
w
3500
3400
33b2
3200
3100
'
ZEOO
25cc
2Y30
WRVEVUMBEPE
Figure 5. Infrared spectra in the 3600-3000- and 2650-2370-cm-' regions for A r / ( D F HF) = 200/1 sample, 90% DF enriched, deposited a t 12 f 1 K following temperature cycles to 14, 20, and 24 K.
+
LJRVENUMBERS
Figure 4. (a) Infrared spectra in the 4000-3600- and 2900-2650-cm-' regions for 40 mmol of A r / ( D F H F ) = 300/1 sample, approximately 75% D F enriched, deposited for 16 h a t 12 i 1 K. (b) Spectrum after warming to 26 f 1 K for 10 min and recooling to 12 K.
+
region exhibits three sets of strong bands, which are due to DF counterparts of the strong bands at 3404, 3244, and 3186 cm-' in Figure lb, and satellite features, which are listed in Table 11. The former region shows the effect of substantial DF substitution in the (HF), species; noteworthy features are a multiplet with eight partially resolved components between 3478 and 3382 cm-I, and a strong 3290-cm-' band and weaker 3192- and 3124-cm-' bands aboue their (HF), counterparts. The product bands in Figure 5 exhibited different growth profiles on stepwise warming of the sample after deposition; the 3412-, 2536-, and 2520-cm-' bands, weak in the original sample, increased on warming to 14 K and
weak new bands appeared at 3290 and 2430 cm-'. Warming to 20 K increased the latter over the former, and the final temperature cycle increased these bands and produced new bands at 3192, 3124, 2388, and 2382 cm-' shown in Figure 5 . Far-infrared spectra were recorded for a number of samples. New product bands are shown at 404, 347.1, 307, 300, 214, 187, and 162 cm-' in Figure 2b from a 200/1 sample with approximately 75% DF enrichment. Warming this sample to 26 K for 10 min increased the 404- and 347.1-cm-' bands 10-fold, produced new satellite bands at 399, 363, and 322 cm-', increased a 310, 307 cm-I doublet at the expense of the 300-cm-' band, doubled the 187- and 162-cm-l bands, and split the 214-cm-I band into a 212.9, 214.9 cm-I doublet with a 4-fold intensity increase. The far-infrared region is illustrated in Figure 6 for this sample from 410 to 150 cm-' and from 1000 to 410 cm-' for the 300/1 sample with 75% DF after sample warming to 26 K for 10 min and recooling to 12 K. The spectrum shows a broad 920-cm-' band which is probably the DF counterpart of the 1250-cm-' band and a broad 830-cm-I band below the analogous (HF), species. A strong band was observed at 498 cm-' with a satellite at 508 cm-', and other mixed isotopic bands in this region are listed in Table 11.
Discussion The matrix absorptions will be assigned to (HF), species and compared to previous studies in order of their generation on sample warming; this order follows increasing displacement (Av,) of the hydrogen-bonded H-F fundamental below the isolated diatomic
FTIR Spectra of (HF), Species in Solid Argon 0 1
1
-
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 429 n
r
-
tentatively assigned to a bent N,--HF--HF species. HF Dimer-(HF),. The next band in order of relative increase in absorbance on sample warming to allow diffusion and association of trapped H F is the sharp, strong, 3825.5-cm-I absorption labeled D. A 1.9 absorbance ratio was observed for this band on sample warming to 21 K (Figure 1) whereas a 7.0 ratio was observed for the sharp 3702.0-cm-I band labeled T. The D band is shifted 42 cm-' from the 3868-cm-I gas-phase band origing assigned to the hydrogen-bonded Hb-F stretch in (HF),
\
\
Figure 6. Infrared spectra in the 1000-410-~m-~ range for a sample of Ar/(DF HF) = 300/1,75% DF enriched, and the 410-150-~m-~range for a similar 200/1 sample after deposition at 12 f 1 K, warming to 26 f 1 K for 10 min, and recooling to 12 K. Horizontal bars connect corresponding (HF), and (DF), species.
+
H F value, which is related to the strength of the hydrogen-bonding interaction. HF Monomer. The strong 3962.7, 3953.8 cm-l doublet is clearly due to H F in solid argon, in agreement with two earlier s t u d i e ~ ,and ' ~ ~a~single ~ D F counterpart was observed at 2895.8 cm-I. The two-site model of Bowers et a1.I9 attributed the 3962.7-cm-l band to the R(0) line of HF undergoing hindered rotational motion and the 3953.8-cm-' band to H F in a matrix site where rotation is quenched. Apparently, these two lines coalesce into a single line for D F at 2895.8 cm-I. In the case of DF the P(1) vibration-rotation line was clearly observed at 2858.1 cm-I, but the HF counterpart at 3877 cm-I increased on sample warming and recooling, which implies slow dissipation of rotational energy to the argon matrix. The 3919.5- and 2876.7-cm-I bands for HF and D F are midway between the R(0) and P ( l ) vibration-rotation line positions in the forbidden Q-branch region, and these lines are assigned to the Q branch of rotating monomers induced by the presence of impurity molecules or lattice imperfections, in agreement with earlier workers. This explanation is reinforced by recent studies of Girardet and Maillard.30 N2--HF. The sharp 3881.5-cm-' band exhibited a growth on sample warming consistent with the formation of a hydrogenbonded complex between HF and another molecule in the matrix sample. A nitrogen-doped experiment was done, and the 388 1S-cm-' band was considerably stronger, and the 3787.7-cm-I band was observed in the initial spectrum ( A = O.l), but the other bands were observed as described for the Ar/HF = 400/1 experiment. The 3881.5-cm-' band is assigned to the H-F fundamental v, in the N2--H-F complex in agrement with Bowers et al.I9 This band exhibits an H F / D F ratio of 1.3637, near the HF/DF monomer value of 1.3654. The microwave spectrum of N2--H-F indicates a linear complex with a comparatively weak hydrogen bond31consistent with the observation of a small displacement (Avs = 72 cm-I) in the H-F fundamental from the 3953.8-cm-I value for H F itself. The 262-cm-I band increased markedly on sample warming and upon addition of N, to the sample, which indicate assignment to the N2--HF species. The splitting into a sharp 263.2 and 260.7 cm-I doublet upon sample warming is reminiscent of similar behavior for the 389-cm-l mode of the linear OC--HF complex in solid argon.32 The 214-cm-I DF counterpart increased markedly and split into a 214.9 and 212.9 cm-I doublet on sample warming. The H F / D F ratio for these doublets (1.2246) supports their assignment to the v I modes for Nz--HF and N2--DF. The 3787.7-cm-I band also increased with added NZ,and its marked growth on sample warming suggests that more than one H F is present. The 3787.7-cm-I band is (30) Girardet, C.; Maillard, D. J . Chem. Phys. 1982, 77, 5941. (31) Soper, P. D.; Legon, A. C.; Read, W. G.; Flygare, W. H. J . Chem. Phys. 1982, 76, 292. ( 3 2 ) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6341.
F
1
which is quite reasonable for a matrix solvent shift. The 3825.5-cm-I band exhibited a DF counterpart at 2803.5 cm-l with a 2808.0-cm-' satellite in mixed DF-HF experiments (Figure 4); however, the 3825.5-cm-l band did not exhibit a mixed isotopic satellite. The 2808.0-cm-I band intensity decreased with increasing DF enrichment as expected for a mixed isotopic species (HF)(DF). Although the 3 8 2 5 - ~ m -band ~ is stronger than expected from a statistical model, which may be a consequence of stronger infrared intensity for (HF), than (DF),, the relative intensities of the 2808.0- and 2803.6-cm-I bands, which are due to Db-F stretching modes in similar deuterium-bonded complexes, are in excellent agreement with the statistical weights of (HF)(DF) and (DF), assuming all mixed dimer has the HF--DF arrangement. This agreement and the lack of a satellite near 3825.5 cm-I demonstrate the following: (a) the dimer structure is bent as in the gas phase6 and not cyclic in solid argon; (b) the deuterium-bonded isotopomer HF--DF is energetically more stable than the hydrogen-bonded isotopomer DF--HF; and (c) the latter less stable mixed dimer structure rearranges to the former more stable structure during dimerization. This rearrangement occurs in a 20 K matrix because the hydrogen-bond energy (4.9 kcal/mol) exceeds the activation energy for rearrangement (tunneling barrier, 0.9 kcal/mol) .I5 The observation of only the HF--DF mixed isotopomer in solid argon is in agreement with both microwave and theoretical The sharp D bands at 3825.5, 2808.0, and 2803.6 cm-' are assigned to the hydrogen- or deuterium-bonded Hb-F or Db-F stretching modes in the HF--HF, HF--DF, and DF--DF dimers, respectively. The small 4.4-cm-I difference between the D-F stretching modes for HF--DF and DF--DF indicates a small amount of vibrational coupling between the two submolecules in the dimer, which is consistent with a relatively weak hydrogenbonding interaction (Av,= 138 cm-'). The hydrogen bond in (HF), is, however, stronger than the hydrogen bond in Nz--HF. The early gas-phase spectrum revealed a substantially weaker 3896-cm-l band, which was assigned to the Ha-F stretching mode in the and the more recent high-resolution study9 located this band origin at 3929 cm-'. The weak matrix band at 3896 cm-' exhibits the proper absorbance ratio on diffusion to be due to the same species as the stronger 3825.5-cm-I band. Another weak band was observed at 3891 cm-I in the HF-DF experiment (90% DF) consistent with little interaction between the Ha-F and Hb-F (or Db-F) vibrations for the HF--HF and HF--DF isotopomers. The weak 3896-cm-I band is accordingly assigned to the Ha-F stretching mode for (HF), in solid argon, which is displaced 33 cm-I from the recent gas-phase rigi in.^ The order-of-magnitude greater intensity for the Hb-F stretching mode is due to hydrogen bonding. The weak 3867-cm-I band attributed by Bowers et all9 to (HF), was not observed in the present studies; this weak band was probably due to an impurity in their system. The 401-cm-l band maintains constant relative intensity with the 3825-cm-I band in experiments with different Ar/HF ratios and on sample warming and exhibits an appropriate H F / D F = 401/307 = 1.306 ratio for a librational motion v l of the Hb-F submolecule in 1, presumably the in-plane libration. The 396-cm-' component is probably due to either a matrix site splitting or the out-of-plane libration. The weaker 322-cm-' satellite of the
430
-
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
307-cm-' band is appropriate for the Db-F librational motion in the species (H,-F)(Db-F); it is noteworthy here that a mixed isotopic satellite was observed only above the v, and vl Db-F submolecule modes in (DF),. The 401-cm-' argon matrix band can be associated with the weak 380-cm-' gas-phase ab~orption,3~ which has been assigned to (HF)2,4allowing for a small solvent shift by the matrix. Although the solvent shifts v, modes to lower energy, vI modes are shifted to higher energy.27 The 400-cm-' assignment to vI of (HF), is lower than 500-600-cm-' values calculated from a force field based on theoretical calculations, which predict a value near 200 cm-' for the lower frequency bending mode." The weak 190-cm-' band appears to track with the 401-cm-' band on sample warming as far as can be determined in this difficult region of the spectrum, and the 190-cm-' band is tentatively assigned to the in-plane Ha-F librational motion in 1. The very low HF/DF = 190/162 = 1.173 ratio indicates a very anharmonic motion, in accord with potential surface calculations,'$ and the observation of a mixed isotopic satellite only at 187 cm-' is consistent with the existence of only the (HaF)(Db-F) mixed isotopomer.6,2' The argon matrix assignments of weak 3896-cm-' and strong 3825.5-cm-' bands to (HF), are in complete agreement with gas-phase infrared absorption ~ p e c t r a , but ' ~ ~in~disagreement ~~~ with assignments based on recent vibrational predissociation spectra.I6 The latter study assigned strong 3878- and 3720-cm-' bands to (HF),. The gas-phase infrared spectrum of Van Huong and Couzi4 shows a strong, broad band near 3860 cm-', consistent with the vibration of hydrogen-bonded H-F, and the 36003800-cm-' region is free of similar strong, broad absorption. Clearly, the 3720-cm-' predissociation band is due to a different (HF), species and not (HF),. Interpretation of the predissociation spectrum requires the (HF), ion fragmentation pattern, which probably includes (HwlFn-,)+ as well as the assumed (HnFwl)+. It is suggested here that the 3720-cm-' predissociation band16 is due to open (HF),, which gives the (H2F)+fragment on ionization. The strong 3702.0-cm-' argon matrix absorption is probably due to the same (HF), species as the 3720-cm-' predissociation band. The present sample-warming studies clearly show that the species reponsible for the 3702.0-cm-l band increases at a greater rate on diffusion of H F (Figure 1) and requires more time to reach a maximum (Figure 3) than (HF),, which further points to (HF), assignment for the 3702.0-cm-' band. HF Trimer-(Hq3. The sharp 3702.0-cm-' band labeled T represents the next generation (HF), species following (HF), based on absorbance growth from two different types of diffusion experiments. Sharp, weaker 3858- and 38 16.0-cm-' bands exhibited the same absorbance growth as the stronger 3702.0-cm-' band. The 3702.0-cm-' band exhibited a DF counterpart at 2717.4 cm-l with a 2719.7-cm-I satellite in mixed DF-HF studies (Figure 4). The 27 19.7-cm-' band absorbance decreased with increasing DF enrichment, suggesting a mixed isotopic species of higher order than (HF),. Sample warming gave absorbance ratios near 6 for these bands and produced a sharp, new 2798.7-cm-l counterpart for the weaker 3816.0-cm-' band. The 3816- and 3702-cm-' bands are assigned to the H,-F and Hb-F stretching modes, respectively, of an open trimer (2) H0-F, Hb
\
F--- Hc-F
2
probably of the trans structure. These bands exhibit HD/DF ratios (3816.0/2798.7 = 1.3635; 3702.0/2717.4 = 1.3623) in near agreement with the diatomic ratio. A weak 3858-cm-' band that followed the 38 16- and 3702-cm-l bands on sample warming in the 400/ 1 experiment is tentatively assigned to the Ha-F mode. The positions of these fundamentals can be rationalized by the (33) Smith, D.F.J . Chem. Phys. 1968, 48, 1429
Andrews and Johnson bonding effect of anothzerH-F submolecule on (HF),. Owing to the substantial fluoride ion affinity of H,-F, the Hb-F fundamental in 2 is shifted considerably below the Hb-F mode in 1, but the terminal H-F submolecule modes in 2 are shifted only slightly below the H-F modes in 1 since the terminal H-F submolecules in 2 have little effect on each other. Six mixed HF-DF species are possible for 2, and two of these can be identified from the spectra (Figure 4). The HF--DF--DF arrangement is eqpbcted to have a spectrum similar to that of (DF),, and the 2719.7-cm-I satellite band is assigned to the Db-F mode. The relative intensity of the 2719.7-cm-' satellite and 2717.4-cm-' main bands in this 75% DF enriched sample indicates that the 2719.7-cm-' band probably arises from two mixed trimers, and the Db-F mode in DF--DF--HF probably contributes to the 2719.7-cm-' band as well. The HF--HF--DF arrangement is expected to exhibit an Hb-F mode near 2, and the 3671.2-cm-' band can account for this mixed trimer. The observation of satellite bands in the DF-HF experiments that can reasonably be assigned to two mixed trimer isotopomers supports the present trimer identification and assignments. The sharp, strong 446.3-cm-' band shows a growth rate on diffusion nearly matching that of the sharp, strong 3702.0-cm-' band, and the 446.3-cm-' band is assigned to the strongest (presumably in-plane) librational mode of the Hb-F submolecule in 2. The librational modes of the Ha-F and H,-F submolecules are expected at lower energy with lower absorbances which prevent their observation. Grouping isotopic satellites is not straightforward for this band due to the beamsplitter break between the all-HF and all-DF components. The observation of isotopic satellites between the 446.3- and 347.1-cm-' bands in the mixed HF-DF experiments indicates more mixing among the librational modes of the open trimer than might be expected. Relative intensities suggest that the stronger 404- and 399-cm-' satellite bands are due to (HF)(DF), and the weaker 363-cm-' satellite band is due to (HF),(DF). The 560-cm-' band also exhibits appropriate diffusion behavior for another v, mode of the open trimer (presumably out-of-plane). The multiplet observed for the 560-cm-' band in mixed HF-DF experiments is consistent with a trimer identification;unfortunately overlapping of multiplets makes specific assignments impossible. Finally, the weaker 497-cm-' band in H F experiments is tentatively assigned to another u1 mode of the trimer. The sharp bands at 3717 and 3618 cm-', which exhibit HF/DF ratios for H-F stretching modes, remain to be assigned. While the diffusion behavior and sample concentration dependence show that these bands are due to multimers in the n = 4 range, these bands do not track with each other in all of the experiments. The relationship between the 2 and 1 spectra discussed above suggests that the latter two sharp bands are due to open tetramers. The broader bands displaced to substantially lower frequency (3427-3 118 cm-') are due to different species with stronger hydrogen bonding. Since ab initio calculations have shown that cyclic trimers and tetramers have stronger total hydrogen-bond energy than their open-structured counterpart^,^^ the broader bands in the 3500-3100-cm-' region are assigned here to cyclic multimers. The observation of maximum intensity in spectrum no. 9 (Figure 3) for the 3702 and 3427, 3404 cm-' bands provides evidence for trimer stoichiometries. The broader 3427- and 3404-cm-' bands are assigned to the strongest H-F stretching mode of the cyclic trimer in two matrix sites or two different ring conformations. The matrix cage clearly affects the structure of the trimer that can be formed; direct condensation at 12 K favors the open structure but sample warming to allow diffusion and association of H F clearly allows the cyclic structure to be formed in a more flexible matrix cage. This is in accord with calculations that predict the latter to be the more stable structure.34 A similar behavior has been found for the less stable HF-HCN and more stable HCN-HF complexes in solid argon.27 The HF/DF = 3427/2536 = 340412520 = 1.351 ratios are appropriate for a strong infrared-active H-F stretching funda(34) Del Bene, J. E.; Pople, J. A. J . Chem. Phys. 1971, 55, 2296
FTIR Spectra of (HF), Species in Solid Argon mental; the mixed H F / D F satellites are complicated as would be expected for cyclic species where the vibrational modes change on partial isotopic substitution. The observation of bands at 2574 and 2560 cm-I, which could reasonably be due to D-F in cyclic (HF),(DF), and bands at 3478 and 3472 cm-I, which could be H-F in cyclic (HF)(DF),, show that the ~yclic-(HF)~ species has another higher frequency H-F stretching fundamental that is not observed here. The present assignment of bands at 3427 and 3404 cm-I to cyclic trimer in solid argon is in reasonable agreement with the 341 0-cm-I peak assigned in the predissociation study16 to cyclic trimer. The stronger 3310-cm-I peak assigned to cyclic trimer in the predissociation study is, however, in the wrong direction for a matrix shift, which suggests that the 3310-cm-I predissociation band might instead be due to cyclic tetramer. HF Tetramel--(HF)r. The above discussion suggests that the sharp 3717- and 3618-cm-l bands are due to two of the four structural isomers34 of the open tetramer. The stabilization of these structures clearly depends upon the formation of the matrix cage to prevent ring closure. The broader 3298-, 3267-, and 3244-cm-I bands, which exhibit similar diffusion behavior with the above sharp bands, are assigned to different structural conformers or matrix sites of cyclic tetramers. The substantially larger Av, for the cyclic tetramer, as compared to the open species, indicates considerably stronger hydrogen bonding in the cyclic species, as shown by ab initio calculation^.^^ The H F / D F = 3244/2421 = 1.340 ratio is compatible with an H-F stretchingmode assignment although it is pointless to attempt to characterize this motion in terms of in-phase and out-of-phase H-F stretching motions since the structural conformations of the cyclic tetramers are not known. It is, however, noteworthy that the 3290-cm-' satellite band, which has appropriate relative intensity to be due to the H-F stretching mode in cyclic (HF)(DF),, is blue shifted from the strongest 3267- and 3244-cm-I ~ y c l i c - ( H F bands. )~ In a sense this is like H F in solid DF, which absorbs near the average of in-phase and out-of-plane H-F motions for solid HF,35and it points to another higher frequency H-F stretching mode for cyclic (HF),, probably in the 3320-3350-cm-' range, which was not observed here. Comparison of the argon matrix cyclic-tetramer bands with the broad gas-phase absorption at 3380 and 3480 cm-l is meaningless since the matrix absorptions are probably band origins and the gas-phase absorptions must involve unresolved combination progressions. However, the predissociation measurements on seeded beams following expansionI6 should also provide band origins. The maxima at 3220 and 3250 cm-I are in very good agreement with the present matrix assignments to cyclic (HF), and the strong predissociation peak at 3310 cm-l, discussed above, is probably due to tetramer instead of trimer, as originally assigned. The predissociation studyI6 suggested different structural forms of cyclic tetramer, which is in accord with the matrix spectrum. The sharp 682-, 694-, and 730-cm-I bands track best with the cyclic tetramer stretching modes, which suggests assignment to librational modes of cyclic tetramers, although the possibility that the sharp superposed bands are due to cyclic trimer and the broad underlying band is due to cyclic tetramer cannot be ruled out. The H F / D F = 682/498 = 1.369 ratio reflects much less anharmonicity than found for the librational modes of the open trimer; similar ratios have been observed for the librational modes of solid HFG3,This is in accord with stronger hydrogen bonding in the cyclic tetramer as indicated by larger Avs values. (HF), and (HF),. The last two broad bands at 3 186 and 3 118 cm-l clearly exhibited different diffusion behavior (Figure l), and the most straightforward assignment of these two bands is to cyclic pentamer and hexamer. These bands are in very good agreement with strong origins at 3200 and 3160 cm-I assigned to cyclic (HF), and cyclic (HF),, respectively, in the predissociation study.I6 The observation of the very strong in-phase H-F stretching mode for H F solid3, at 3065 cm-I with a much weaker out-of-phase H-F (35) Kittelburger, J. A.; Hornig, D. F. J . Chem. Phys. 1967,46, 3099.
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 431 stretching mode higher at 3404 cm-I suggests that the strong matrix pentamer and hexamer bands have a weaker, unobserved mode at higher frequency. This is in accord with the observation of mixed isotopic satellite bands blue shifted (Table 11) from the (HF), (n = 5,6) absorptions. Finally, the HF/DF = 3186/2382 = 1.338 ratio for the pentamer is appropriate for an H-F stretching fundamental vibration. The remaining broad bands near 730, 865, and 1250 cm-I in solid argon exhibited growth on diffusion appropriate for high multimer species. A comparison with band maxima near 720, 840, and 1200 cm-I in the gas-phase absorption study' is striking, and the solid has broad absorptions near 1200 and 962 cm-' and a sharp band at 552 cm-I in this region.35 It is suggested that the broad matrix bands are each due to different H-F librational modes which overlap for the cyclic pentamer and hexamer. The 865-cm-I band grows more relative to the 850-cm-' band in Figures 1 and 3, which suggests that the former could be due to the hexamer and the latter due to the pentamer; a like association can be offered for the 1250-cm-' band, which shows more pronounced growth on sample warming in Figure 1 than the band at 1240 cm-I. The higher frequency H-F librational modes and the larger displacement of the H-F stretching modes for the hexamer indicate stronger hydrogen bonding for the hexamer, in agreement with theoretical calculation^.^^
ConcIusion Condensation of dilute mixtures of H F and argon at 12 K produced strong, sharp H-F stretching absorptions in the 40003600-cm-l range, broader bands in the 3500-3 100-cm-I region, and H-F librational bands in the 1250-150-cm-' range, which exhibited marked and different H F concentration dependences. The product absorptions changed by different fractions on temperature cycling the sample, and they reached maximum absorbances in different times as the sample temperature was increased slowly over a 6-h period. These observations provide convincing evidence for the stepwise association of H F giving a numer of different (HF), species. Isotopic substitution with DF in 60-90% enrichments provided (DF), counterparts for these absorptions and satellite absorptions for mixed (HF),(DF), species. The H-F stretching fundamentals at 3896 and 3825 cm-I for the dimer are in agreement with recent high-resolution studies: allowing for a small matrix solvent shift, and observation of only the (HaF)(D,-F) mixed dimer arrangement is in agreement with microwave studiese6 An open trimer has been characterized by a sharp H-F stretching fundamental at 3702 cm-I, and evidence has been presented for open tetramer absorptions in this region. Four distinctly different groups of absorption bands have been observed in the 3100-3500-~m-~region; the increasing width and displacement of these bands is consistent with the formation of four distinctly different cyclic-(HF), species (n = 3, 4, 5, 6) with increasing hydrogen-bond strength, which is in agreement with ab initio calculation^.^^ The controlled diffusion of H F in solid argon provides a mechanism for the stepwise association of H F giving all multimers in the n = 2, 3, 4, 5, and 6 range. Since a distinct difference was observed between the (HF), and (HF), absorptions at 3186 and 31 18 cm-I, the absence of similar bands in the 3000-3 100-cm-I region strongly suggests that higher oligomer species are not formed in these studies; if higher oligomers cannot be formed by association of HF in solid argon at 20-28 K, it is unlikely that higher oligomers will be stable in the gas phase at higher temperatures. The observation of a strong 3290-cm-I band in DF-enriched experiments that can reasonably be assigned to the H-F stretching fundamental in cyclic (HF)(DF), provides a measure of the average hydrogen-bond strength in the cyclic tetramer, which is near the v, mode for H F in the strong hydrogen-bonded (CH,),CO--HF complex at 3302 cm-' in solid argon.,, Accordingly, the cyclic-(HF), species (n = 4, 5,6) are strong hydrogen-bonded com(36) Johnson, G. L.Ph.D. Thesis, University of Virginia, Charlottesville, VA, 1984.
432
J . Phys. Chem. 1984, 88, 432-440
plexes, based on the displacement of their strongly absorbing H-F fundamentals from the diatomic value. We gratefully for this research by the National Science Foundation, a Sesquicentennial Associateship from the University of Virginia, a Visiting Fellowship from the Science and Engineering Research
Council (U.K.), and a Fulbright Senior Research Fellowship, the assistance of B. J. Kelsall and R. T. Arlinghaus with several experiments, a Beckman spectrum of H F species in solid argon from B, S, Ault to with the present observations, and word processing of the manuscript by Southampton University. Registry No. HF, 7664-39-3; DF, 14333-26-7; Ar, 7440-37-1.
Matrix Isolation Infrared Spectroscopic Study of Sulfur Dioxide-Amine Complexes Craig S . Sass and Bruce S . Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: April 8, 1983)
The reaction products of sulfur dioxide and a variety of methyl-substituted amines were studied in argon and nitrogen matrices, after formation through both single and twin jet deposition. In each case, a 1:l complex was observed and characterized by a shift in the infrared absorptions of the SO2subunit in the complex. The antisymmetric stretching mode shifted to lower energies upon complexation, from 1350 cm-' for free SO2, to 1338 cm-' in the SO2.NH3complex, to approximately 1270 cm-' for the S02.(CH3),N complex. A similar trend was observed for the symmetric stretching mode of the SO2subunit. Band assignments were confirmed by the use of '*O labeled SO2,and normal coordinate calculations were used to fit the observed frequencies to a set of force constants. The primary S-0 stretching force constant F, was found to decrease linearly with increasing base strength of the amine, indicating the SO2 is serving as a a* acceptor. The infrared spectra were consistent with the theoretically calculated geometry, but an insufficient number of product bands were observed to fully confirm the calculation. In addition, the 1:2 complex SOz.2NH3 was observed spectroscopically for the first time.
Introduction Molecular complexes between Lewis acids and bases have been known for years,'J and complexes involving strong Lewis acids such as BF3 are well characterized.,~~Sulfur dioxide, SO2,is a much weaker Lewis acid, but nonetheless is known to form weakly bound charge-transfer complexes with nitrogen-containing While some thermodynamic data have been obtained for these complexes, little is known about their structure, with the exception of the S02.N(CH3),complex, for which an X-ray crystal structure has been determined.g Several groups have attempted to obtain low-temperature infrared spectra of these adducts, with varying degrees of success. Histasune and co-workers10 reported the solid-state spectrum of a sample formed through a sequential condensation of NH, and SOz, followed by annealing. Reahal" and Nord12 have each reported argon matrix spectra of the SO2.", complex, but substantial points of disagreement exist in all of these studies and no firm conclusions were reached. One group attempted', to obtain gas-phase spectra of the SO2/", system, but only detected thionyl imide, NHSO, and the bisulfate salts. Concurrent with these experimental efforts, a number of theoretical studies have been carried out, on the S02.NH3 complex
by Schaefer and co-workers,14and on the methyl-substituted amine complexes by Kollman et a l l s These groups-reach quite similar conclusions, namely, that the plane of the SO, molecule was approximately perpendicular to the S-N bond, which was itself colinear with the C, axis of the amine. This calculated structure agrees well with the crystal structure determined9 for the trimethylamine (TMA) complex, SO,-TMA. The calculated binding energies ranged from 11 to 15 kcal/mol, which agrees well with the experimental value for SOz.TMA of 10 kcal/mol. In addition, a 1:2 adduct is known for the S 0 2 / N H 3system, and has been postulated for several other amines. While a considerable effort has been directed toward the study of amine complexes of SO,, in part because of their importance in atmospheric chemistry, substantial gaps remain in our understanding of these adducts. The matrix isolation technique has proven to be an excellent technique for the study of relatively weakly bound complexes, and has been applied to both protic acid-base systems16-18as well as to Lewis acid-base systems, particularly those involving NH, c o o r d i n a t i ~ n . ' ~With ~ ~ ~ the considerable interest in these molecular complexes, and the need for a thorough investigation of the SO,.amine complexes, a study was undertaken to isolate and spectroscopically characterize these complexes in both argon and nitrogen matrices.
(1) Jensen, W. G. "The Lewis Acid-Base Concepts, an Overview"; Wi-
Experimental Section All of these experiments in this study were conducted on a conventional matrix isolation system which has been described
ley-Interscience: New York, 1980. (2) Niedenzu, K. Ado. Chem. Ser. 1964, No. 42. (3) Swanson, B.; Shriver, D. F. Inorg. Chem. 1970, 9, 1406. (4) Amster, R. L.; Taylor, R. C. Spectrochim. Acta 1964, 20, 1487. (5) Hull, A. E. J . A m . Chem. SOC.1931, 53, 2598. (6) Grundnes, J.; Christian, S. D. J . A m . Chem. SOC.1968, 90, 2239. (7) Landreth, R.; de Pina, R. G.; Heicklen, J. J . Phys. Chem. 1974, 78, 1378. (8) Scott, W. D.; Lamb, D. J . A m . Chem. SOC.1970, 92, 3943. (9) van der Helm, D.; Childs, J. D.; Christian, S. D. J . Chem. SOC.,Chem. Comm. 1969, 887. (10) Hisatsune, I. C.; Heicklen, J. Can. J . Chem. 1975, 53, 2646. (1 1) Reahal, A. S. Thesis, University of Salford, 1981. (12) Nord, L. J . Mol. Struc. 1982, 96, 27. (13) Hata, T.; Kinumaki, S. Nature (London) 1964, 203, 1378.
0022-3654/84/2088-0432$01.50/0
(14) Lucchese, R. R.; Haber, K.; Schaefer, H. F. J . A m . Chem. SOC.1976 -98. - ,7617. --
(15) Douglas, J. E.; Kollman, P. A. J . A m . Chem. S O ~1978, . 100,5226. (16) Auk, B. S.; Pimentel, G. C . J . Phys. Chem. 1973, 77, 1649. (17) Ault, B. S.; Pimentel, G. C. J . Phys. Chem. 1973, 77, 57. (18) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . A m . Chem. SOC.1982, 104, 6180. (19) (a) Auk, B. S. Inorg. Chem. 1981, 20, 2817. (b) McNair, A. M.; Auk, B. S . Ibid. 1982, 21, 1762. (20) (a) Ritzhaupt, G.; Devlin, J. P. J . Phys. Chem. 1977, 81, 521. (b) Nelander, B.; Nord, L. Ibid. 1982, 86, 4375.
0 1984 American Chemical Society
