J. Phys. Chem. 1990,94, 4851-4855
4851
Infrared Matrix Isolation Studies of Hydrogen Bonds Involving C-H Bonds: Alkenes with Selected Bases Mei-Lee H. Jeng and Bruce S. Auk* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45220 (Received: November 28, 1989; I n Final Form: February 8, 1990)
Hydrogen-bonded complexes of ethene and substituted ethenes with strong bases have been isolated and characterized for the first time in argon matrices at 15 K. Coordination of the alkenic hydrogen to the electron donor was evidenced by distinct red shifts of the C-H stretching mode in the infrared spectrum. These shifts ranged from 10 to 150 cm-' and are considerably less than those observed previously for the corresponding alkynic complexes, suggesting a distinct but quite weak interaction. The perturbed ethenic hydrogen bending modes and carbon-halogen stretching modes were also observed for a number of systems. The larger the number of halogens on the substituted ethene, the larger the shifts, while fluorinated ethenes gave rise to larger shifts than their chlorine counterparts. Finally, trans-l,2-dichloroetheneconsistently gave rise to larger shifts with a given base than did the corresponding cis compound.
Introduction Hydrogen bonding plays a significant role in chemistry and biology and consequently has generated considerable interest over the years.l Many studies have been directed toward a more complete understanding of this molecular interaction. While the majority of these studies have emphasized relatively strong hydrogen bonds, to fully characterize hydrogen bonding the exploration of weak hydrogen bonds is essential. The ability of C-H bonds to act as proton donors in a hydrogen bond has generated substantial i n t e r e ~ t , ~given - ~ the comparable electronegativities of carbon and hydrogen. However, the hydbridization of the carbon atom and the substituents on the molecule are known to affect the acidity of the C-H bond. Recent studies from this l a b ~ r a t o r y ~have - ~ characterized hydrogen-bonded complexes of a series of alkynes with a range of bases. The weak hydrogen-bonding interaction was characterized by a red shift of 20-300 cm-I for the alkynic hydrogen stretching motion. The magnitude of this shift was consistent with the gas-phase proton affinity of the base subunit and the Hammett substituent constants of the substituted alkynes. It is generally accepted that sp2-hybridized carbon atoms are much less acidic than sp-hybridized (alkynic) carbon atoms. Recently, Klemperer and co-workersIo,II reported that C2H2serves as a proton donor to H 2 0 , while C2H4serves as a proton acceptor from HzO. At the same time, several hydrogen-bonded complexes of the haloforms have been observed,'-l2J3 indicating that the attachment of electron-withdrawing substituents to the molecule increases the acidity of the C-H bond. One report14 in 1963 provided evidence that the haloethenes can form hydrogen bonds to strong bases such as pyridine in CCI4 solution, but little additional research has been directed toward the hydrogen-bonding ability of this class of compounds. The matrix isolation t e c h n i q ~ e l ~has , ' ~ been frequently used to study weakly bound c~mplexes,'~-'~ including hydrogen-bonded ( I ) Pimentel, G.C.; McClellan, A . L. The Hydrogen Bond; W. H . Freeman: San Francisco, 1960. (2) Sapse, A . M.; Jain, D. C. Chem. Phys. Lett. 1986, 124, 517. (3) Frisch, M. J.; Pople, J. A.; Dei Bene, J . Chem. Phys. 1983, 78, 4063. (4) Truscott, C. E.; Ault, B. S. J . Phys. Chem. 1984, 88, 2323. (5) Manceron, L.; Andrews. L. J . Phys. Chem. 1985, 89, 4094. (6) DeLaat, A . M.; Ault, B. S. J . Am. Chem. SOC.,1987, 109, 4232. (7) Jeng, M.-L. H.; DeLaat, A. M.; Ault, B. S. J . Phys. Chem. 1989.93, 3997. (8) Jeng, M.-L. H.; Auk, B. S . J . Phys. Chem. 1989. 93, 5426. (9) Jeng, M.-L. H . ; Auk, B. S. J . Phys. Chem. 1990, 94, 1323. (IO) Peterson, K. I.; Klemperer, W. J . Chem. Phys. 1984, 81, 3842. ( I I ) Peterson, K. I.; Klemperer, W. J . Chem. Phys. 1986, 85, 725. (12) Creswell, C . J.; Allerd, A . L. J . Am. Chem. SOC.1962, 84, 3966. (13) Creswell, C. J.; Allerd, A . L. J . Am. Chem. SOC.1963, 85, 1723. (14) Allerhand, A.; Schleyer, P. v. R . J . Am. Chem. SOC.1963,85, 1715. ( 1 5) Craddock, S.; Hinchliffe, A. Matrix Isolation; Cambridge University Press: New York, 1975. (16) Andrews, L. Annu. Reo. Phys. Chem. 1971, 22, 109. (17) Ault, B. S. J . Am. Chem. SOC.1983, 105, 5742. (18) Ault, B. S. Rev. Chem. fntermed. 1988, 9, 233.
c~mplexes.&*-~"-~~ Infrared spectroscopy remains a very sensitive experimental tool for the detection of hydrogen bonding in that the criteria for hydrogen bond formation are the distinct shift, broadening, and intensification of the proton donor hydrogen stretching mode. Consequently, a study was undertaken to probe hydrogen bonds involving alkenic hydrogens, emphasizing interactions with bases stronger than H 2 0 and bases that are not readily capable of proton donation.
Experimental Section A conventional matrix isolation apparatus, which has been described p r e v i o ~ s l ywas , ~ ~ used for all of the experiments in this study. The reagents employed were CzH4, (CH3)3N, NH3, ( C H 3 ) z 0(all Matheson), (CD,),N (Merck), cis-dichloroethene, trans-dichloroethene, 1,I-dichloroethene (all Aldrich), 1,I-difluoroethene, trifluoroethene (both PCR), trichloroethene (Fisher), ( C H 3 ) 2 C 0 (Baker), and (CH3),P (Alfa). All reagents were subjected to one or more freeze-thaw cycles at 77 K prior to sample preparation. Argon was used as the matrix gas throughout and was used without further purification. Many samples were deposited in the twin-jet mode, in which the two reagents were each diluted with argon in separate vacuum lines and deposited simultaneously on the 15 K cold window. In a few cases, additional single-jet experiments were carried out in which the two reagents were premixed in a single vacuum line and diluted with argon, and the mixture was then deposited. A deposition rate of approximately 2 mmol/h for 20-24 h was used throughout. For some pairs of reagents, final spectra were recorded on a Perkin-Elmer 983 infrared spectrometer at 2-cm-I resolution; for the remaining systems a Nicolet IR 42 FTIR spectrometer was employed, with 1-cm-' resolution.
Results Prior to any codeposition experiments, blank experiments were conducted with each of the alkenes; the resulting spectra were in good agreement with literature spectra.2630 For all of the bases employed in this study, blank spectra had already been recorded (19) Sass, C. E.; Auk, B. S. J . Phys. Chem. 1986, 90, 4533. (20) Andrews, L. J . Mol. Srruct. 1983, 100, 281. (21) Ault, B. S.; Pimentel, G . C. J . Phys. Chem. 1973, 77, 57. (22) Johnson, G.; Andrews, L. J . Am. Chem. SOC.1983, 105, 163. (23) Barnes, A . J. J . Mol. Struct. 1983, 100, 259. (24) Barnes, A . J.; Paulson, S . L. Chem. Phys. Lerr. 1983, 99, 326. (25) A u k B. S. J . Am. Chem. Soc. 1978, 100, 2426. (26) Herzberg, G . D. Molecular Spectra and Molecular Structure; Van Nostrand, New York, 1975; Vol. 2. (27) Bernstein, H . J. Can. J . Res. B 1950, 28, 132. (28) Bernstein, H . J. Can J . Chem. 1954, 32, 1044. (29) Mann, D. E.; Acquista, N.; Plyler, E. K. J . Chem. Phys. 1954, 22, 1586. (30) Smith, D. C.; Nielson, J. R.; Claassen, H . H . J . Chem. Phys. 1950, 18, 326.
0022-3654/90/2094-485 1$02.50/0 0 1990 American Chemical Society
4852
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 -~ ___ ____ _____-._
Jeng and Ault
__
i
I
I A
ArfC2F3H
a
Ar'
55.:
8-7
753209
Figure 1. Infrared spectra obtained after the codeposition of samples of Ar/C2F3H with either a sample of A r / N H , (upper traces) or Ar/ (CH,),P (lower traces) compared to blank spectra of each reagent alone
3'30
3332
35C
9X
852
337
E h E 2 G V (cm-1'
in solid argon.
Figure 2. Infrared spectra of the products of the codepasition of CH2CF2 with (CH&N (upper traces) and with (CH3),P (lower traces) into argon matrices, compared to blank spectra of each reagent alone in solid argon.
in earlier ~ t u d i e s . ~Additional .~ blank experiments at different concentrations were nonetheless carried out when needed. Trifluoroethene Reactions. The twin-jet codeposition of a sample of Ar/C2F3H = 500 with a sample of Ar/(CHJ3N = 500 gave rise to a new broad, weak absorption at 3010 cm-l, a weak band at 3061 cm-I, and a triplet centered at 838 cm-'. There was also some apparent intensification of parent C-F modes at 1142, 1161, and 3025 cm-I. Additional experiments were conducted at higher concentrations, with similar results. The codeposition of C,F3H with N H 3 at different concentrations led to a sharp product band at 3063 cm-l, with shoulders at 3076 and 3082 cm-I. In addition, weak bands were observed at 838, 844,920, and 984 cm-I, as well as an apparent increase in the intensities of parent bands at 1142, 1161, 1361, and 3025 cm-I. C2F3H was also codeposited with (CH3)20,both at concentrations of 500/1 in argon, and led to a moderately intense feature at 3080 cm-l that overlapped a weak parent absorption at 3092 cm-I. A considerable increase in intensity of parent bands at 3025 and 3146 cm-l was noted. I n addition, a distinct doublet was observed at 808 and 8 12 cm-I, along with a weak feature at 762 cm-I. Increasing the concentration of Ar/(CH,),O to 100 while holding that of the C2F3H sample constant gave rise to more intense and distinct product bands (except that band at 762 cm-I) and a shoulder at 3085 cm-l. Further changes in concentration led to similar results. The codeposition of a sample of Ar/C2F3H = 500 with a sample of Ar/(CH,),CO = 500 or 200 resulted in a weak, sharp feature at 3086 cm-l along with a multiplet centered near 805 cm-I. Again, an apparent increase in the intensity of the parent absorptions at 1142 and 1161 cm-' was noted. C2F3Hwas also deposited with (CH,),P at several different concentrations; an intense triplet was detected at 3067, 3074, and 3082 cm-l, along with a medium-intensity feature at 3141 cm-', a doublet at 794, 798 cm-I, and a band at 918 cm-I. Also, increased intensity of the 1 142 and 1 16 I cm-' parent bands was noted. Representative spectra of the C2F3H reaction products are shown in Figure 1. I ,I -Difluoroethene Reactions. The codeposition of a sample of Ar/CH,CF, = 500 with Ar/(CH3),N = 500 resulted in weak
new features at 3012, 3028, and 3149 cm-I, the latter with a shoulder at 3140 cm-I, as shown in Figure 2. At lower energies, distinct new absorptions were noted at 846, 916, and 960 cm-l, with a shoulder on the 846-cm-' band, at 850 cm-l. Also, a dramatic increase in the intensity of the parent (CH3),N mode near 1270 cm-I was noted. Increasing the concentration of CH2CF2while holding that of (CH,),N constant led to more distinguishable product bands. Increasing the concentration of (CH,),N while holding CH,CF2 constant at 500/1 in argon led to a considerable increase in intensity of the product bands at 960, 3012, and 3140 cm-' but not the 3028- and 3149-cm-I bands. The twin-jet codeposition of CH2CF2with NH,, each diluted in argon, gave rise to a broad, medium-intensity feature at 3027 cm-' with a shoulder at 3036 cm-' and two weak bands at 3013 and 3151 cm-'. In the lower energy region, a multiplet centered at 839 cm-' was observed, along with distinct new absorptions at 958 and 984 cm-I; a shoulder at 1273 cm-I was noted on a nearby CH2CF2parent absorption. Increasing the concentration of either reagent led to similar and more intense product bands. The upper traces of Figure 3 show representative spectra of this system. CH2CF2was codeposited with (CH3)20,each at a concentration of 500/1 in argon, resulting in a medium-intensity new absorption at 3040 cm-l and a weak band at 3154 cm-l. At lower energies, a distinct doublet at 836, 845 cm-' was observed, along with a sharp feature at 955 cm-I. Some intensity variation of several parent absorptions was also noted. The infrared spectra of this system are shown in the lower traces of Figure 3. Increasing the concentration of Ar/(CH,),O to 200/1 while holding that of CHzCF2constant led to an increase in the relative intensities of product bands to parent bands A sample of Ar/CH,CF, = 500 was codeposited with a sample of Ar/(CH,),CO = 500, leading to new absorptions at 836, 916, 955, 1219, 3040, and 3155 cm-I, the last band with a shoulder at 3146 cm-I. A considerable variation in intensity of parent CH,CF2 near 1280 cm-I was noted, along with new shoulders on these bands. Increasing the concentration of acetone gave more distinct and intensified product absorptions. 1, I -Difluoroethene was also codeposited with trimethylphosphine at different con-
-
Hydrogen Bonds Involving C-H Bonds
A=500~1
J---G500!:
A=500!1 8=500/1
A
ArlCH2CF2
a
Ar/NH3
I " , '
3100
'
I
300C
950 E NER G Y
C Ar/(CH3)20 " " " ~ ' " ' ~ ~ ' 900 853 800
(cm-1)
Figure 3. Infrared spectra of the products arising from the twin-jet codepsition of samples of Ar/CH2CF2with a sample of Ar/NH, (upper traces) and A r / ( C H 3 ) 2 0 (lower traces) compared to blank spectra of each reagent alone in solid argon.
centrations in argon, resulting in a new band of medium intensity at 3030 cm-I, along with a weak band at 3151 cm-I. At lower energies, weak absorptions were noted at 833,916,958, and 1281 cm-', as shown in Figure 2. Trichloroethene Reactions. Twin-jet codeposition of a sample of Ar/C2CI3H = 500 with a sample of Ar/(CH3),N = 500 gave rise to a single, very weak feature at 3010 cm-l. Increasing the concentration of each sample led to a slight increase in this 3010 cm-I, as well as the appearance of a weak feature at 3038 cm-I. The codeposition of C2CI,H with (CD,),N in both the single-jet and twin-jet modes at several different concentrations led to a distinct product band at 2976 cm-I. The twin jet codeposition of C2CI3H with N H , into argon matrices gave rise to a strong new absorption at 3010 cm-' along with bands at 840, 870, 876, and 984 cm-l. An increase in the N H , concentration led to intensification of all of the product bands. C2CI3Hwas also codeposited with (CH3)20,at concentrations of 500/ 1 and 200/ 1 in argon, respectively. An intense new doublet was observed at 3048, 3052 cm-I, as well as shoulders at 838,922, and 1094 cm-l on nearby parent absorptions. The codeposition of C2C13Hwith (CH,),P at concentrations of 500/1 and 200/1 in argon, respectively, led to a shoulder at 839 cm-' and a distinct triplet at 3019, 3026, and 3049 cm-I. Dichloroethene Reactions. The codeposition of a sample of Ar/cis-l,2-dichloroethene= 200 with a sample of Ar/(CH3),N = 200 produced weak product absorptions at 728,732,840, 1306, 1312, 1319, 3013, and 3020 cm-'. Similar experiments were conducted at higher concentrations of each reagent, and led to the growth of the above product bands. When cis-C2C12H2was codeposited with NH, at several concentrations, a dramatic enhancement was noted in a band attributed to the dimer of C2CI2H2 at 3046 cm-I. In addition, weak new bands were observed at 710, 725, 840, and 1313 cm-I. The codeposition of a sample of Ar/cis-C2ClzH2= 500 with sample of Ar/(CH3)20 = 500 resulted in a new band of medium intensity at 3063 cm-l with a shoulder at 3072 cm-l. Weak features were also noted at 722, 842, 918, 1092, and 1314 cm-',
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4853 as well as some broadening of parent absorptions. A similar sample of Ar/cis-C2C12H2was codeposited with a sample of A r / ( C H J 2 C 0 and led to a distinct shoulder at 3073 cm-l on the low-energy side of the parent absorption at 3082 cm-I. In addition, new product absorptions were noted at 702, 722, 842, and 1290 cm-I, along with some broadening and/or shifting of the parent absorption near 715 cm-I. trans- 1,2-Dichloroethene was codeposited with (CH,),N into argon matrices and led to the observation of a weak, broad feature at 3004 cm-I, a weak band at 1216 cm-l, and some intensification of a parent band at 909 cm-I. trans-C2H2CI2was also codeposited with NH, into argon matrices; in these experiments no distinct new product bands were observed. However, a weak parent band at 3034 cm-' was greatly intensified, and a parent mode at 825 cm-' was broadened. When the NH, concentration was increased, the enhancement of the 3034-cm-I band was yet more apparent, and a distinct shoulder was noted at 818 cm-I. The codeposition of 1,l-dichloroethene with NH, at different concentrations in argon resulted in a sharp, strong absorption at 3109 cm-.', along with intensity enhancements to the parent bands of l,l-C2H2CI2near 600, 906, 1090, and 3010 cm-I. These changes were more dramatic with an increase in N H , concentration. The codeposition of l,l-C2H2CI2iwith (CH,)3N led to a distinct product band at 3109 cm-I, which increased when the concentration of trimethylamine was increased. Finally, 1,l -dichloroethene was codeposited with acetone into an argon matrix at concentrations of 250/1 and 200/1 respectively; a distinct absorption was noted at 3123 cm-' to the red of the parent absorption at 3 133 cm-'. Ethene Reactions. The twin-jet codeposition of a sample of Ar/C2H4 = 500 with a sample of Ar/(CH,),N = 100 gave rise to weak features at 3010 and 3038 cm-I, along with a broadening of the parent C-H bending modes. In addition, a considerable increase in the intensity of a shoulder at 936 cm-I on the parent band at 946 cm-I was noted, as well as a new, weak band at 968 cm-I. Increasing the concentration of C2H4 while keeping that of (CH,),N constant resulted in the same set of product bands, with about the same intensities. Representative spectra of the products arising from the codeposition of C2H4 with (CH3),N are shown in Figure 4. C2H4 and NH, were codeposited in a number of experiments, in both the single-jet and twin-jet modes at different concentrations. An apparent broadening of the parent mode near 3109 cm-I, with maxima appearing as shoulders at 3095 and 3107 cm-l. In addition, a weak feature was observed at 3014 cm-l, along with a broadening of the -CH2 bending mode. The twin-jet codeposition of C2H4 with (CH,),O did not give rise to any distinct new features, but some broadening and changes in relative intensity of parent adsorptions was noted. The single-jet codeposition of this pair of reagents at a concentration of Ar/C2H4/(CH,),0 = 500/ 1/ 1 gave rise to distinct product absorptions at 954 and 3095 cm-I, as shown in the lower traces of Figure 4. Tables I and I1 summarize the product bands positions for all of the systems described above.
Discussion Evidence for an interaction between the alkenes and the bases employed in this study comes from the comparison of the infrared spectra of the codeposition experiments with the spectra of the isolated acid and base molecules. In the codeposition experiments, new absorptions were noted that could not be ascribed to either parent species. The observed product bands can be grouped into four sets: (1) those occurring near and to the red of the parent alkenic C-H stretching mode (3000-3100 cm-I); (2) those falling near and to the blue of the parent - C H 2 or - C H X bending mode; (3) those occurring near and to the red of one or more of the parent C-X stretching modes; (4) those occurring for some systems quite near vibrational modes of the base. The nearness of the product absorptions to modes of the parent species suggests that the alkene and base subunits are perturbed in the product, rather than addition, elimination, or rearrangement products. For each system, the most intense and distinct product absorption was a broad, often
4854
The Journal of Physical Chemistry, Vol. 94, No. 12, I990
Jeng and Ault
TABLE I: Band Positions (cm-I) for the Hydrogen Stretching Mode, vS, for Hydrogen-Bonded Alkene Complexes in Argon Matrices base
(CH3)3N alkene
NH3
us
us
3038 3010 2976b 3012 3004 3016" 3109
3109 C2F3H 3156 CZCI3H 3107 C2H2F2 3102 Z-C~H~CI, 3107 c - C ~ H ~ C I ~ 3083 C2H2C12 3133 C2H4
(CH,)2O A% 14 93 97 75 73 37 24
us
C2HZC PAd
71 146 131 90 I03 67 24 I60 225
3095 3063 3010 3027 3034 3046 3109
(CH3)2CO
1)s
3095 3080 3050" 3040 3063 3123
115
204
(CH3)J'
us
AUs
us
AU$
3086
70
82 76
3040
62
3074" 303 I a 3030
3073
IO
14
76 57 62 20 IO 73 192
72
54 197
67 227
" Mean value. *Frequency taken from complex with (CD,),N. 'Taken from refs 6, 7. 9. dProton affinity of the base, in kcal/mol. TABLE 11: Band Positions (cm-') for the In- and Out-of-Plane Hydrogen-Bending Modes" for Hydrogen-Bonded Alkene Complexes in Argon Matrices alkene
base (CH,),N
Y
946 968
A1
22
NH3
(CH,)20 (CHh2CO (CH3)3P
954
8
6 948 960 958 955 955 958
A6
Y
12 IO 7 7 IO
803 846 839 836 836 833
..lY
Y
43 36 33 33 30
752 838' 838 810' 805' 796
A?
86 86 58 53 44
6 I298 1312' 1313 1314
A6 14
15 16
6
Ay
y
699 728 725 722 722
A6
735 29 26 23 23
823 795 77V 772'
88 60 43 37
"6 refers to in-plane, y refers to out-of-plane. *Taken from refs 6, 7, 9. "Mean value
moderately intense absorption to lower energies of the parent C-H stretch, hereafter referred to as us. The distinctness and intensity of the product bands, in general, argues that the interaction between the subunits is a directional and specific interaction, rather than a nondirectional, dispersive interaction (van der Waals). All of the above features are precisely the infrared spectral characteristics of hydrogen bonding,' and consequently in each system studied here, the product absorptions can be assigned to a hydrogen-bonded complex. The intensities of the product absorptions were directly proportional to the concentrations of each of the reagents, and the same products were observed at low concentrations (1000/1/1) as well as at the highest concentrations employed here (400/1/1). These observations point to a single product species, and one with 1 :1 stoichiometry. Thus, the product species described here are identified as the isolated 1 : I hydrogen-bonded complex of the appropriate alkene and base; this represents the first systematic report of hydrogen bonding involving alkenes in an inert environment. Since the us product band for each of the ethene systems fell quite close to the C2H4dimer, assignment of these bands to a hydrogen-bonded product must be taken as tentative. However, the observation of additional product bands in other spectral regions supports observation of ethene complexes as well. The observable most commonly used in the characterization of hydrogen bonded systems is the shift of us, i u s ; this is often taken as a measure of the strength of interaction. The values of Aus observed here, 10-150 cm-l, are substantially smaller than those observed for many hydrogen-bonded systems; for alkynic hydrogen bonds, shifts of 30-300 cm-' were observed, while for systems involving strong acids such as H F and HCI, shifts of 1OC-1000 cm-' or more have been reported. These values indicate that while distinct hydrogen bond formation does occur, the complexes are more weakly bound than those reported previously, in agreement with the expected low acidity of sp2-hybridized 1 carbon atoms and the results from Klemperer's For two of the alkenes studied here, definitive band assignments in the C-H stretching region were not available. For C2H2F2, several different reported the antisymmetric C-H (31) Scherer, J. R.: Overend, J. J . Chem. Phys. 1960, 32, 1720 (32) Freeman, J. M.;Henshall, J. Can. J . Chem. 1969, 47, 935. (33) McKean. D.E. Speclrochim. Acta Parr A 1975. 3 / A . 1167
7 -
____
1
! L r T T 7 J \
310C
3000
'3cio
950
3
E NE R G Y (crn-1)
Figure 4. Infrared spectra, over selected spectra regions, obtained after the codeposition of C2H4with (CH,),N (upper traces) and (CH3),0 (lower traces) compared to blank spectra of each reagent alone in an argon matrix.
stretching mode at positions between 3100 and 3170 cm-I. In the present study, parent bands were observed at 3 102 and 3 I68 cm-', the latter appearing only at high concentrations. Consequently, the 3102-cm-' band was assigned to this mode; assignment of the
Hydrogen Bonds Involving C-H Bonds 3168-cm-' band to the antisymmetric C-H stretch would also have yielded values of Au, that were out of line with all of the other shifts reported here. Likewise, for C2F3H early studies placed the C-H stretch at 3 102 cm-I; more recent studies29have placed this band at 31 56 cm-I; this latter value was used here. In addition, for several systems (e.g., (CH3)3N with C2CI3H), us would be expected to fall underneath the parent (CH3)3N C-H stretching modes; in these cases, (CD3),N was employed to shift the parent bands to lower energy (near 2100 cm-I). With these considerations, Table I with all of the Au, values can be derived. A second characteristic spectral change upon hydrogen-bond formation is a shift to higher energy of the proton donor bending modes, with shifts that in general are smaller than Au,. Two such modes are anticipated for each complex, and in-plane bend (6) and the out-of-plane bend (7). For the complexes cis-C2H2CI2 and CH2CF2,two such shifted modes were observed, while for the complexes of C2F3Hand C2H4, only one was observed. These band positions are collected in Table 11. The magnitudes of these shifts are less than those observed for the alkynic hydrogen-bonded complexes, again indicating that the alkene hydrogen-bonding interaction is weaker than for analogous alkynic complexes. At the same time, the shifts of y were consistent with the shifts in u, reported here. N o ~ o k has ) ~ reported a roughly linear relationship between Au, and AT for a number of -OH systems, with AuOH/AyOH= 5.8. This ratio is much smaller for the current alkene hydrogen-bonded systems, which may reflect the weakness of the interaction. At the same time, this relationship is only empirical and there is no particular theoretical reason to expect this ratio to be constant over a wide range of hydrogen-bonded systems. For a given base, the general ordering of Aus is C2F3H> C2CI3H > I , 1-C2F2H2= trans- 1 ,2-C2CI2H2> cis-l ,2-C2C12H2> 1 , l C2C12H2> C2H4. Not surprisingly, the greater the number of halogens on the substituted ethene, the greater the shift, given the electron-withdrawing nature of the halogen family, which should increase the acidity of the alkenic hydrogen. Likewise, a fluorine atom is more electron withdrawing than a chlorine atom36and hence gave larger shifts for comparable compounds (e& C2X3H). For cis- and trans-l,2-dichloroethenea halogen is attached directly to the carbon bearing the acidic hydrogen, while for I,l-dichloroethene this is not the case. Thus, the frequency shifts for the complexes of 1,2-dichloroethene were larger then those of 1,l-dichloroethene. It is interesting that complexes of trans- 1,2-dichloroethene gave rise to larger shifts than did cis- 1,2-dichloroethene, despite the (35) Novok, A. Sfrucf. Bonding 1974, 18, 177. (36) Chapman, N . B.: Shorter, J. Correhfion Analysis in Chemistry; Plenum Press: New York. 1978; Chapter 10.
The Journal of Physical Chemistry, Vol. 94, No. 12, I990 4855 fact that the trans compound has no permanent dipole moment. This is in keeping with the results described by Allerhand and SchleyerI4 for a series of chloroalkenes in CCI4 solution. The explanation for this trend is not clear but may lie in orbital overlap in the trans compound which enhances the electron-withdrawing capability of the chlorine atom substituents. Additional modes of both the acid and base subunit in the complex were perturbed by hydrogen bond formation. For many of the complexes, either a distinct absorption or a shoulder appeared on the low-energy side of one of the C-X stretching modes. Shifts on the order of 3-10 cm-I were observed, due to slight electron density rearrangement upon hydrogen bond formation. Intensity enhancement was noted for a number of parent bands, without the resolution of a distinct product absorption in the region. This indicates that the shift of this mode in the complex was sufficiently small that the product absorption fell within the envelope of the parent absorption. At the same time, the intensification of the band suggests that the absorption coefficient of this mode in the complex is greater than the same mode in the isolated parent. This may be due to increased polarity or polarization upon hydrogen bond formation and consequently increased dipole moment derivative during the vibration. Finally, for the complexes of C2H4 a weak feature was observed around 3010 cm-l. This is assigned to the totally symmetric C-H stretching mode, which should be formally activated in a hydrogen-bonded complex but which should be of low intensity. In this study, a number of bases were employed, and a systematic trend was noted for a given alkene through the range of bases. In each case, complexes of (CH3)3Ngave the largest shifts and complexes of (CH3)2C0the smallest, a trend consistent with the studies of alkynic hydrogen bonds. Interestingly, shifts for complexes of (CH3),P were smaller than analogous complexes of (CH3)3N,even though the two bases have comparable gas-phase proton affinities3' This was rationalized earlier in terms of hard/soft acid/base (HSAB) theory,38an explanation that appears to hold here as well. Finally, it should be noted that in a number of (CH3)3Nexperiments, several additional product bands were observed. These came exactly where absorptions of the corresponding NH3 complex were located and can be attributed to residual N H 3 impurity in the vacuum manifold where (CH3)3Nwas being handled.
Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation through Grant C H E 87-21969. (37) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. ReJ Data 1984, 13, 695.
(38) Pearson, R. G.; Songstad, J. J . Am. Chem. SOC.1967, 89, 1827.
