J. Phys. Chem. 1992, 96, 4288-4294
4288
framework of a two-body collision picture.'* Aechtner et al.38 observed a nonmonotonic temperature dependence of the dephasing time for the v 1 mode of CS2 in the pressure range from 1 bar to 4 kbar and a temperature range of 160-450 K. Oehme et ala" observed a nonmonotonic temperature dependence of the line width for furan over a large temperature range and we were able to reproduce their data at ambient pressure up to the boiling temperature. However, it is not clear from their experimental description whether the line widths were recorded at constant pressure or even exclusively in the liquid phase. In our pressure experiments, a nonmonotonic temperature dependence at constant pressure is not observed for any of the modes in the temperature range studied except for the 3092-an-' peak. At constant pressure, the 3092-cm-l band first narrows and then begins to broaden a t the end of the density range studied. However, at constant density which is required in the Strekalov and Burshtein theory,I8 all of the modes studied showed monotonic temperature dependencies within the experimental error. Furthermore, our data indicate that the linewidth narrowing observed by Oehme et al.I7 as the temperature was increased to room temperature is caused mainly by density changes rather than being attributable solely to temperature effects. V. Conclusions In this work, our discussions included three basic topics concerning liquid furan, namely, the temperature and pressure responses of various vibrational modes in terms of their line widths and peak maxima, the temperature dependence of isotropic line widths and the vibrational dephasing mechanisms interpreted by the modified Schweizer-Chandler model. It was found that the line widths and frequency shifts of all the modes analyzed show definite monotonic trends along either isotherms or iscchores, but (38) Aechtner, P.; Fickenscher, M.; Laubereau, A. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 399.
with varying degrees. Among the three different types of vibrational modes studied we observe that the C-H stretching modes have the strongest temperature and pressure responses, followed by the bending modes and the ring modes. On the basis of these phenomenologicalobservations, we expect that the 3092-an-' peak, which was previously assigned as an overtone of the 1556-cm-l ring vibration, is instead a fundamental C-H stretching vibration. From the density dependence of the frequency shifts for the C-H breathing mode at 3159 cm-l, our results showed that the behavior of this mode in furan is similar to those of other organic liquids.32 This implies that, in liquid furan, the repulsive and attractive frequency shifts are also dominated by packing effects rather than by the intermolecular potential. The IBC model modified by anharmonic effects overestimates the line width due to vibrational dephasing. The original Schweizer-Chandler model also failed to describe the density and temperature dependences of our measured line widths. However, the SC model with a parametric correction can be used to fit our data quite well and resulted in two main qualitative results. First, the vibration-rotation coupling contribution to the line-width broadening decreases with decreasing temperature. Second, the necessity of including the soft-core correction is more important at the lower temperatures due to the different temperature responses of the critical diameter and the effective diameter of the furan molecule. In addition to the vibrational dephasing, the temperature and pressure dependences of the depolarized Rayleigh and Raman spectra would also be useful in providing information about the molecular interactions, such as the reorientational and collision-induced effects, in liquid furan. This will be discussed elsewhere in the near future.
Acknowledgment. This work was supported in part by the National Science Foundation under grant N S F C H E 90- 17649. We express our thanks to Professor K.-L. Oehme for suggesting the high-pressure study of liquid furan. Registry No. Furan, 110-00-9.
Matrix Isolation Study of the Reaction of B,H, with CH,OH: Spectroscopic Characterizatlon of Methoxyborane, H,B =OCH, John D. Carpenter and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: November 18, 1991; I n Final Form: February 4, 1992)
Merged jet copyrolysis of mixtures of Ar/B2H6and Ar/CH30H followed by trapping into a cryogenic matrix has led to the formation, isolation, and characterization of methoxyborane, H2B--VCH3. The same product was observed in lesser yield following merged jet copyrolysis of B2H6 and (CH3)20;ethoxyborane was detected following copyrolysis of mixtures of B2H6and CzHSOHin argon. Of the 18 fundamentals of methoxyborane, 17 were observed, including the boron-oxygen stretch at 1358 cm-l ("B). The position of this mode, compared to a range of known compounds, suggests considerable double bond character in methoxyborane. Additional products were detected at higher pyrolysis temperature and higher relative methanol concentration, including methane, boroxin, and dimethyl ether. These observations collectively lead to an overall mechanism for the reaction of B2H6with CH30H.
(1) Stock, A.; Massenez, C. Chem. Ber. 1912, 45, 3539.
(2)
Muetterties, E. Boron Hydride Chemistry; Academic Press: New
York, 1975. (3) Lane, C. F. Chem. Rev. 1976, 76, 773.
(4) Hirayama, M.; Shohno, K. J . Electrochem. SOC. 1975, 122, 1671. ( 5 ) Hyder, S. B.; Yep, T. 0. J . Electrochem. SOC.1976, 123, 1721. (6) A d a m , A. C.; Capio, C. D. J . Electrochem. SOC.1980, 127, 399. (7) Carpenter, J. D.; Ault, B. S. J . Phys. Chem. 1991, 95, 3502.
0022-365419212096-4288%03.00/0 0 1992 American Chemical Society
Matrix Isolation Study of the Reaction of B2Hs with C H 3 0 H The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4289 TABLE I: Band Positions (cm-I) and Assignments for Argon Matrix Isolated Methoxyborane 12C 13C CDq assignment 0.30
0.20
0.10
0.00
-0.10
40.0
Figure 1. Infrared spectrum, from 400 to 4OOO cm-I, of an argon matrix containing the products of the 300 OC merged jet pyrolysis of a mixture of Ar/B2H6 with a mixture of Ar/CH30H.
led to formation of both the 1:l borane adduct and the H2 elimination product, for mono- and dimethylamine. Reaction of B2Hs with trimethylamine yielded only the 1:l adduct H3B-N(CH3)3. The generality of this reaction has not yet been established, nor the nature of the H2 elimination process (1,l elimination followed by a hydrogen shift, or a 1,2 elimination). Kawashima and co-workers9pyrolyzed diborane with methanol and reported the microwave spectrum of methoxyborane, H2B==OCH3, the oxygen analogue of methylaminoborane. While the microwave spectrum of H2B=OCH3 is known, the infrared spectrum has yet to be reported. The matrix isolation technique'*12 is well suited to the study of reactive intermediates; the results of Kawashima et al. suggest that the pyrolysis of B& and C H 3 0 H in argon followed by matrix trapping should yield methoxyborane. In addition to interest in this unusual species itself, the observation of methoxyborane should shed light on reaction pathways for these and related reactions. Consequently, a matrix study was undertaken to isolate and characterize the products of the pyrolysis of and CH30H, (CH3)20and C2HSOH.
Experimental Section The matrix isolation experiments in the present study were all carried out on conventional equipment which has been described.13 Dimethyl ether (Matheson) and di(methy1-d3) ether (MSD Is@ topes, 99% D) were introduced into the vacuum line as gases and purified by freeztthaw cycles at 77 K. CH30H (HPLC grade, Fisher), CD30D, CD30H, CH30D (all MSD Isotopes, 99%), and C2HSOHwere introduced into the vacuum system as the vapor above the liquid after repeated freeze-thaw cycles at 77 K. B2Hs (Matheson) was introduced into the vacuum line as a 1% mixture in argon and diluted further with argon to the desired ratio. Samples of h / B & and Ar/CH30H (or derivative) were prepared in separate manifolds. While a few twin jet experiments were conducted, in the majority of the experiments the merged jet mode was employed. Here, the deposition lines connecting the two manifolds to the cold cell were joined together (merged) through an ultratorr tee. The two samples were permitted to flow together in a '/4-in.-o.d. copper tubing for the 145 cm from the tee to the cold window, before being sprayed onto the cold window. Heating tape was wrapped around the last 90 cm of the merged region and could be heated as high as 400 OC. Samples were pyrolyzed in this manner and then deposited directly onto the cold window, as was done previously in the B2H6/amine studies.'J Samples were deposited between 6 and 24 h at a rate of 2 mmol/h before final scans were recorded on a Nicolet IR 42 Fourier transform infrared spectrometer at 1-cm-' resolution. Normal
2994 2878 2575 2565 2487 2482 1506 1471 1383 1358
2984 2264 A' 2874 2087 2574 2575 2564 2562 2484 2486 2480 2477 1504 1096 1469 1107 1379 1444 1354 1402 1269 1266 1263 1252 916 1162 1154 901 1115 1107 1045 1045 1045 1036 1032 1032 878 1002 991 983 872 993 2956 2951 2246 A" 1489 1486 1137 1183 1175 950 905 905 905 895 894 894 589 585 557
v I , CH, antisymmetric stretch v2, CH, symmetric stretch v,, BH2 antisymmetric stretch (IoB) v,, BH2 antisymmetric stretch ("B) v4, BH2 symmetric stretch (IoB) v4, BH2 symmetric stretch ('IB) v5, CH, antisymmetric deformation v6, CH, symmetric deformation v7, B=O stretch ('OB) v7, B=O stretch ("B) v8, BH2 symmetric bend (loB) us, BH2 symmetric bend ("B) v9, CHI rock ul0, CH3 rock Y , ~ , BH2 rock (I0B) v l I , BH2 rock ("B) v12.C-0 stretch ('OB) v I 2 ,C - 0 stretch ("B) vI4, CH, antisymmetric stretch vis, CH, antisymmetric deformation q 6 , CHI rock ~ 1 7 ,BH2 out-of-plane wag (IoB) ~ 1 7 ,BH2 out-of-plane wag ("B) v I 8 ,torsional vibration
TABLE II: Additional Product Bands (cni') and Assignments following Pyrolysis of B,HJCH,OH and Related Mixtures '2C l3C CD, assignment 3008 3000 methane 3026 methane 2137 1296 1287 methane 1305 1000 methane 2608 2608 boroxin 2608 1419 1419 boroxin 1419 1406 1406 boroxin 1406 1395 1395 boroxin 1395 1380 1380 boroxin 1380 1218 1218 boroxin 1218 1209 1209 boroxin 1209 91 1 91 1 boroxin 911 298 1 2242 2986 dimethyl ether 2878 2190 dimethyl ether 2890 2827 2057 dimethyl ether 2821 1154 dimethyl ether 1460 1160 1063 dimethyl ether 1172 1083 928 dimethyl ether 1098 920 824 dimethyl ether 926 2.00
s
1.8-
d
i.00-
I
0.8-
Figure 2. Infrared spectrum, over the spectral region 1200-1600 cm-', of the products of the 300 O C merged jet pyrolysis of a mixture of Ar/B2H6 with a mixture of Ar/CH,OH.
(8) Carpenter, J. D.; Ault, B. S. J . Phys. Chem. 1991, 95, 3507. (9) Kawashima, Y.; Takeo, H.; Matsumura, C. J . Mol. Specrrosc. 1986, 116, 23. (10) Craddock, S.; Hinchliffe, A. Matrix Isolation; Cambridge University
coordinate calculations were carried out on the University of Cincinnati computer, using a program provided by the National Research Council (Canada).
Press: New York, 1975. (1 1) Hallam, H. Vibrational Spectroscopy of Trapped Species; Wiley: New York, 1973. (12) Andrews, L. Annu. Rev. Phys. Chem. 1971, 22, 109. (13) Ault, B. S. J . Am. Chem. SOC.1978, 100, 2426.
Results Prior to the codeposition of B2& with any of the above reagents, blank spectra were recorded of each reactant alone in solid argon.
4290 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
0.00
L O
, ,
I
,
ish
I
IB
XAMHWBERS
Figure 3. Infrared spectrum, over the spectral region 1800-2000 cm-I, of the products of the merged jet pyrolysis of a mixture of Ar/B2H6 with a mixture of Ar/CH,OD.
Blank experiments were conducted with the deposition line a t several different temperatures to provide direct comparison to the codeposition pyrolysis experiments. In every case, the blank spectra were in good agreement with literature ~ p e c t r a . ' ~ - ~ ~ B2H6 + CH30H. The twin jet codeposition of this pair of reagents led to no new infrared absorptions. Similarly, merged jet codeposition of B2H6 with C H 3 0 H a t a total concentration of 1000/1/1, with the deposition line at room temperature led to no new absorptions. However, when the merged jet deposition line was heated to 150 OC and the gaseous mixture pyrolyzed prior to matrix deposition, a number of new infrared absorptions were observed. When the deposition line was heated to 300 OC, all of the listed product bands were significantly more intense, and additional product bands were observed; all new absorptions are listed in Tables I and I1 and shown in Figures 1 and 2. Parent absorptions of C H 3 0 Hwere completely absent, and those of B2H6 were very weak. When the concentration was changed to either 1000/1/2 or 1000/2/1 under similar conditions, the overall product yield was unaffected. When the pyrolysis temperature was increased to 340 OC, parent absorptions were completely absent, while the listed product bands remained, although with reduced intensity. Additional experiments were conducted at a variety of temperatures to verify the reproducibility of all of the observed product bands. In the highest yield experiments (around 300 OC), the most intense product absorption at 1358 cm-' was nearly 1.6 absorbance units. B2H6 CD30H. When these two reagents were codeposited in the merged jet deposition mode at 300 OC, a series of new infrared absorptions was observed, as noted in Tables I and 11. These bands were reproducible over a series of experiments under different temperature and concentration conditions. B2& CH30D. The merged jet deposition of diborane with CH30D with pyrolysis at 300 OC and a concentration ratio of 1OOO/ 1/ 1 in argon yielded all of the product bands observed earlier in comparable experiments with B2H6 and CH30H. In addition, new product absorptions were observed at 865, 977, 984, 1143, 1868, 1890, and 1909 cm-I; the latter two bands are shown in Figure 3. All of these product bands were reproduced in several experiments over a range of temperatures and concentrations. B2H6 + CD30D. When these two reagents were codeposited in the merged jet mode at 300 OC, all of the observed product bands were comparable to those of the B2H6/CD30Hexperiments (and are reported in Tables I and 11). In addition, new infrared absorptions were noted at 970, 993, 1120, 1230, 1878, 1890, and 1909 cm-'. All of these absorptions were observed in the same relative amounts in a series of experiments over a range of temperatures and concentrations. BZH6+ '%X30H. Several experiments were conducted with this pair of reagents in the merged jet mode at 300 OC with concentrations ranging from lOOO/ 1/2 to 1000/2/ 1. Numerous product bands were observed, in a manner analogous to those
+
+
(14) Lassegues, J. C.; Grenie, Y.; Forel, M. T. C. R. Seances Acad. Sci., Ser. B 1970, 271, 421. (15) Barnes, A. J.; Hallam, H. E. Trans. Faraday SOC.1970, 66, 1920. (16) Barnes, A. J.; Hallam, H. E. Trans. Faruday Soc. 1970, 66, 1932.
Carpenter and Ault TABLE III: Band Positions (cm-I) and Tentative Assignments for Ethoxyborane' band tentative band tentative position assignment position assignment 2995 -CZHS* 1287 BH2 symmetric bend ("B) 2915 -CZHs* 1166 -CZHS* 2584 BH2 antisymmetric stretch 1123 -CZHJ* 1112
