5426
J. Phys. Chem. 1989, 93, 5426-5431
by +0.334 kcal/mol; however, our deviation at u = 28 from ref 24 is only +0.163 kcal/mol (see Table 111). Variations in different reported RKR values of rmaxdo not appear significant but can be substantial when expressed in terms of corresponding energy. For instance, for u = 18 of HF, ref 15 reports r,,, = 2.555 8, (re = 0.917 17 A), and ref 23 reports rmax= 2.563 (re = 0.91681 A); conversion of these distances to energy values gives -4.55 and -4.07 kcal/mol, respectively. This discrepancy of 0.5 kcal/mol is greater than our average deviations in most cases. Similar extreme examples are provided by comparing the RKR data collected in ref 15 with the more recent values from the references given in Table I1 for u = 19 of N 2 (energy difference 0.6), u = 14 of N 2 (energy difference 1.6), u = 22 of O2 (energy difference 0.2), etc. While we do not doubt that the accuracy of a priori functions will be improved further, the present function appears to be approaching the limits of reliability of the RKR points in some instances. At large r, high-order polynomials in 1/ r are superior to exponential functions for curve fitting to known RKR points. We find that our function also shows increasing values of percent error beyond 95-98% of dissociation, overestimating stability in that region. Absolute errors are small in the region of the long-range potential. Computer evaluation of the energy at exactly re with fractional powers of n can lead to an invalid argument in the log routine. Negative powers of n have not been examined. Equation 2 can be made to produce potential wells wider that those of the Morse function by using C = -2 - (1 + u ) ~ .
Acknowledgment. We thank L. R. Zavitsas for her assistance in the establishment of the RKR data base and her advice on properties of various mathematical functions. Partial support of this work by the Committee on Research of the Brooklyn Campus of Long Island University and by the Research School of Chemistry of the Australian National University is gratefully acknowledged.
Appendix The zero of energy is defined as V ( a ) ,the energy of the two separated atoms. The thermodynamic bond dissociation energy, Do, is the energy difference between the lowest vibrational level and V(m), expressed as a positive number. Similarly, De is the depth of the potential well. De and Do differ by the zero-point energy; the approximation De = Do + 0 . 0 0 1 4 3 ~is~generally accurate to better than 0.07 kcal/mol. The equilibrium vibrational frequency we, is related to the observed vibrational frequency, v, by we = v 2 (w,xe) cm-’, where ( w & ~ ) is the “anharmonicity”. The force constant is given by k, = (we/ 1303)2p, where p is the reduced mass in atomic mass units. De, we, and k, are theoretical constructs. In terms of the experimentally obtainable molecular parameters Do and v, the following approximations may be used if necessary: D,’ = Do + 0.00143~,k’= ( ~ / 1 3 0 3 ) ~ p ,= 8.486(k’/00)’f2, kN = kf/Do, and rN = re/Dd. The use of k’together with Do in calculating p leads to partial cancellation of errors.
+
Infrared Matrix Isolation Study of Hydrogen Bonds Involving C-H Bonds: Substituent Effects Mei-Lee H.Jeng and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: January 20, 1989)
The matrix isolation technique combined with infrared spectroscopy has been employed to isolate and characterize hydrogen-bonded complexes between a series of substituted alkynes and several oxygen and nitrogen bases. Distinct evidence for hydrogen bond formation was observed in each case, with a characteristic red shift of the hydrogen stretching motion us. Shifts between 100 and 300 cm-’ were observed, the largest being for the complex of CF3CCH with (CH3)3N. The perturbed carbon-carbon triple bond stretching vibration was observed for most complexes, as was the alkynic hydrogen bending motion. Attempts were made to correlate the magnitude of the red shift of us with substituent constants for the different substituted alkynes; a roughly linear correlation was found with the Hammett u parameter. Lack of correlation Aus with either u1 or nR alone suggests that both inductive and resonance contributions to the strength of the hydrogen-bonding interaction are important.
Introduction Numerous studies have been carried out to study hydrogen bonding as a consequence of the very significant role this interaction plays in chemical, physical, and biological processes.’-3 Infrared spectroscopy has emerged as one of the most effective experimental tools for the study of hydrogen-bonding interactions, since hydrogen bond formation gives rise to distinct, readily identifiable spectral features.’ The matrix isolation technique4“ combined with infrared spectroscopy has been applied very effectively by a number of researchers to characterize hydrogenbonded The most frequent participants in hydrogen bonding are the highly electronegative elements nitrogen, oxygen, and fluorine. Although the electronegativities of carbon and hydrogen are similar, the possibility that a C-H group may serve as a proton donor has generated substantial experimental and theoretical It is generally accepted that the proton-donating ability, the acidity of a C-H group, is dependent on the hybrid-
* Author to whom correspondence should be addressed. 0022-3654/89/2093-5426$01.50/0
ization of the carbon as well as the substituent groups in the molecule. In previous reports from this l a b o r a t ~ r y , hydro~~.~~ ( I ) , Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, 1960. (2) Vinogradov, S. N.; Linnell, R. H. The Hydrogen Bonding; Van Nostrand-Reinhold: New York, 1971. (3) Kollman, P. J . Am. Chem. SOC.1977, 99, 4875. (4) Craddcck, S.; Hinchliffe, A. Matrix Isolation; Cambridge University Press: New York, 1975. (5) Andrews, L. Annu. Rev. Phys. Chem. 1971, 22, 109. (6) Hallam, H., Ed. Vibrational Spectroscopy of Trapped Species; Wiley: New York, 1973. (7) Ault, B. S. Acc. Chem. Res. 1982, 15, 103. (8) Truscott, C. E.; Ault, B. S. J . Phys. Chem. 1985, 89, 1741. (9) Ault, B. S.; Pimentel, G . C. J . Phys. Chem. 1973, 77, 57, 1649. (10) Andrews, L. J . Mol. Struct. 1983, 100, 281. (11) Johnson, G. L.; Andrews, L. J . Am. Chem. Soc. 1982, 104, 3043. (12) Barnes, A. J . J . Mol. Strucf. 1983, 100, 259. (13) Sapse, A. M.; Jain, D. C. Chem. Phys. Lett. 1966, 124. 517. (14) Frisch, M. J.; Pople, J. A.; Del Bene, J. E. J . Chem. Phys. 1983, 78, 4063. ( 15) Truscott, C. E.; Ault, G. S. J . Phys. Chem. 1984, 88, 2323. (16) Manceron, L.; Andrews, L. J . Phys. Chem. 1985, 89, 4094.
0 1989 American Chemical Society
Matrix Isolation Study of Hydrogen Bonds gen-bonded complexes of 1-alkynes with a number of bases containing oxygen and nitrogen donor atoms have been reported. Trends were noted in the degree of perturbation of the hydrogen stretching frequency as a function of the proton affinity of the base, and the shift of this mode was consistent with the gas-phase aciditiesz6of the alkynes. Substituent constants have been widely and successfully used in the correlation of equilibria and reaction rates, including inductive, resonance, and steric terms. The Hammett u value measures the sum of the inductive and resonance terms, although it is at times convenient to separate out the individual contributions. Taft27,28and others29 have utilized division into inductive and resonance terms, u = u1 + uR. Many previous infrared studies of hydrogen bonding have correlated the shift of the hydrogen stretching mode with the strength of interaction,' which in turn might well correlate with substituent constants for a series of related acids. It would be of interest to examine such correlations, over a wide range of substituents on the acid, to more fully understand the hydrogen-bonding interaction. Consequently, a study was undertaken to isolate and characterize the hydrogen-bond complexes of a series of substituted 1-alkynes with a set of reference bases in argon matrices.
Experimental Section All of the experiments described in this study were carried out on a conventional matrix isolation system which has been described p r e v i o ~ s l y . ~The ~ reagents employed in this study were 3,3,3trifluoropropyne, CF3CCH (PCR), propargyl chloride, CHzC1CCH, methyl propargyl ether, CH30CH2CCH, 3-butyn-2-one, CH,C(O)CCH (all Aldrich), (CH&O, NH3, (CH3),N (all Matheson), (CH3)2C0(Baker),'and CH3CN (Fisher). Monochloroacetylene, ClCCH, was synthesized by the method of Bashford, Emeleus, and B r i ~ c o e . ~All ~ were subjected to one or more freeze-thaw cycles at 77 K prior to sample preparation. Argon and nitrogen were used as the matrix gases throughout and were used without further purification. In a typical experiment, the two reactants were each diluted with argon to the desired ratio in separate vacuum manifolds, and codeposited onto the 17 K cold window for 20-24 h at approximately 2 mmol/h from each line. Final spectra were recorded on a Perkin Elmer 983 infrared spectrometer at a resolution of 2 cm-l. Several samples were then annealed to 32 K and recooled to 17 K, and an additional spectrum was recorded.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5427
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Figure 1. Infrared spectra, over selected spectral regions, of the products arising from the codeposition of CF,CCH with (CH,)20 and with (CH&CO into argon matrices, compared to blank spectra of each reagent alone in solid argon.
Results prior to any codeposition studies, blank experiments were carried out on each of the reactants employed here. All of the resulting spectra were in good agreement with available literature spectra,3z44 as well as with spectra recorded previously in this labo, ' , , , - I " '
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(17) Peterson, K. I.; Klemperer, W. J. Chem. Phys. 1986, 85, 725. (18)Peterson, K. I.; Klemperer, W. J. Chem. Phys. 1984, 81, 3842. (19) Fraser, G.T.; Lovas, F. J.; Suenram, R. D.; Nelson, D. D.; Klemperer, W. J. Chem. Phys. 1986,84, 5983. (20) Fraser, G.T.; Leopold, K. R.; Klemperer, W. J. Chem. Phys. 1984, 80. 1423. (21) Fraser, G. T.;Leopold, K. R., Nelson, D. D.; Tung, A.; Klemperer, W. J. Chem. Phys. 1984, 80, 3073. (22) Bach, S.B. H.; Ault, B. S. J. Phys. Chem. 1984, 88, 3600. (23)Paulson, S.L.; Barnes, A. J. J. Mol. Struct. 1982, 80, 151. (24)DeLaat, A. M.; Auk. B. S. J. Am. Chem. SOC.1987. 109, 4232. (25)Jeng, M. L. H.; DeLaat, A. M.; Auk, B. S. J. Phys. Chem., in press. (26) Bartmess, J. E.; Scott, J. A,; McIver, R. T. J. Am. Chem. SOC.1979, 101, 6046. (27)Taft. R. W.J. Am. Chem. SOC.1957. 79. 1945. (28j Taft; R. W. Steric Effects in Orgonic'Chemistry; Newman, M.S . , Ed.; Wiley: New York, 1956;Chapter 13. (29) Ehreson, S.Prog. Phys. Org. Chem. 1964, 2, 195. (30) Auk, B. S.J. Am. Chem. SOC.1978, 100, 2426. (31) Bashford, L. A,; Emeleus, H. J.; Briscoe, V. A. J. Chem. SOC.1938, 1358. (32) Herzberg, G. Molecular Spectra and Molecular Structure; Van Nostrand: New York, 1945;Vol. 2, 290. (33) Sanssey, J.; Lamotte, J.; Lavalley, J. C. Spectrochim. Acto, Purr A 1976, 32A, 763. (34) Evans, J. C.; Nyquist, R. A. Spectrochim. Acta 1963, 19, 1153.
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Figure 2. Infrared spectra obtained after the twin jet codeposition of samples of Ar/CF3CCH with samples of Ar/(CH3)3N (upper traces) and with samples of Ar/CH,CN (lower traces) compared to blank spectra of each reagent alone in solid argon.
ratory. CF3CCH Reactions. The twin jet codeposition of samples of Ar/CF,CCH = 500 with Ar/(CH,),O = 500 gave rise to an (35) Sanborn, R. H.Spectrochim. Acta, Part A 1967, 23A, 1999. (36) Seth-Pawl, W. A.; Tollenaere, J. P.; Meeusen, H.Spectrochim. Acta, Part A 1974, 30A, 193.
(37) Rogstad, A.; Cyvin, S.J. J. Mol. Struct. 1974, 20, 373. (38)Crowder, G. A. Spectrochim. Acto, Part A 1973, 29A, 1885. (39) Lassegues, J. C.; Grenie, Y.; Ford, M. T. C. R. Acad. Sci Paris, Ser. E. 1970, 271, 421. (40) Mann, R. H.;Dixon, W. B. J. Chem. Phys. 1972, 57, 792. (41)Schriver, L.; Schriver, A,; Perchard, J. P. J. Chem. SOC.,Faraday Trans. 2 1985, 81, 1407. (42)Goldfarb, T. D.; Khare, B. N. J. Chem. Phys. 1967, 46, 3379. (43) Milligan, D. E.; Hexter, R. M.J. Chem. Phys. 1961, 34, 1009. (44) Reding, F. P.; Hornig, D. F. J. Chem. Phys. 1951, 19, 594.
5428 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
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