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Infrared study of intramolecularly hydrogen bonded aromatic carbonyl containing compounds in various solvents
VIBRATIONAL SPECTROSCOPY ELSEVIER
Vibrational
Spectroscopy
12 ( 1996) 93- 102
Infrared study of intramolecularly hydrogen bonded aromatic carbonyl containing compounds in various solvents R.A. Nyquist *, C.L. Putzig, T.L. Clark, A.T. McDonald Dow Chemical
Company,
Analytical Received
Sciences Laboratory, 30 May
1995; accepted
I897F
Building,
23 December
Midland,
MI 48667,
USA
1995
Abstract The intramolecularly hydrogen bonded OH group sterically alters the spatial distance between the sites of solute-solvent interaction, and the larger spatial distance causes a weaker solute-sovent interaction. Intramolecular hydrogen bonding also causes the carbonyl group to be relatively less basic which decreases the strength of an intermolecular hydrogen bond formed between the solute and the acidic proton of the solvent. Therefore, there is a smaller vC=O . * * HO frequency decrease compared to vC=O for similar compounds not intramolecularly hydrogen bonded in going from solution in Ccl, to solution in CHCl,, or (CH,),SO. Keywords;
Intramolecularly
H-bonded
aromatic
carbonyl
containing
1. Introduction In this laboratory, we have obtained structural information from IR and Raman studies on solutesolvent interactions [l-26]. In our present study of intramolecularly hydrogen bonded carbonyl containing compounds, we were interested in determining how the OH group affects interaction of the C=O group with different solvents. The solvent acceptor number (AN) was developed by Gutmann, and AN is reported to be a measureof solvent electrophilicity. The AN value is defined as a dimensionlessnumber related to a relative chemical shift of 31P in (C,H,),P=O in that particular solvent, with hexane as a reference solvent on one hand, and
* Corresponding 0924.2031/96/$15.00 PII SO924-203
author. Copyright 1(96)00005-7
0 1996 Elsevier
Science
compounds
(C,H,),P=O . * SbCl, in 1,2-dichloroethaneon the other, to which the acceptor numbers0 and 100 have been assigned[27]. The AN values of a solvent are thought to be useful in predicting vibrational group frequency data of chemicals in a particular solvent, and this study tests the validity of this concept.
2. Experimental Infrared spectra were recorded with the Nicolet 710 FT-IR system which is purged with dry nitrogen in order to exclude ambient air from the sample compartment. The compounds used in this study were purchased from Aldrich or Merck, and were studied without further purification. Carbonyl compounds were prepared as 1 g/100 ml in Ccl, solution and in CHCl, solution. Mole% solutions of CHC13/CCl, were preparedusing stock solutions of
B.V. All rights reserved
94
R.A. Nyquist
Table 1 Infrared data for 2-hydroxyacetophenone
et al. / Vibrational
1 wt./vol.%
Solvent
iC=O...HO,
Hexane Diethyl ether Methyl t-butyl ether Toluene Benzene Carbon tetrachloride Carbon disulfide 1,2-Dichlorobenzene Acetonitrile Nitrobenzene Benzonitrile Methylene chloride Nitromethane t-Butyl alcohol Chloroform Dimethyl sulfoxide Isopropyl alcohol Ethyl alcohol Methyl alcohol
1649.2
in various
cm-i
Spectroscopy
vc=o
0.0 3.9 2.5
1646.7 1644.7 1644.4
1646.2 1645.5 1645.1
3.0
18.9
4.5 8.0 6.7
6.2
20.4 8.2 29.1
5.9
23.1 19.3 33.5
6.9 9.9 3.0
37.1 41.3
3.7 4.1
2.2
each carbonyl compound so that within experimental error the concentration of the solute is constant in each of the mole% CHCl,/CCl, solutions. For example, 1 ml samples of the 1 g/100 ml Ccl, stock solutions were placed separately into ten glass botTable 2 Infrared data
for methyl
salicylate
1 wt./vol.%
in various
tles and 1 ml samples of the 1 g/100 ml CHCl, stock solution were placed separetely into nine glass bottles using calibrated micro syringes. Then, to each of the 1 ml 1 g/100 ml carbonyl compound in Ccl, solution samples we added a calibrated volume of
solvents
Solvent
vc=o
Hexane Dimethyl ether Methyl t-butyl ether Toluene Benzene Carbon tetrachloride Carbon disulfide 1,2-Dichlorobenzene Acetonitrile Nitrobenzene Benzonitrile Methylene chloride Nitromethane t-Butyl alcohol Chloroform Dimethyl sulfoxide Isopropyl alcohol Ethyl alcohol Methyl alcohol
1685.6
0.0
0.0
1682.3
3.9
3.3
1682.2 1679.9 1679.4 1682.5
3.4 5.7
1680.4
1679.2 1680.1 1680.1 1679.4 1678.7 1679.3 1682.1 1678.3 1675.3
1681.8 1681.1 1680.2
* * * HO, cm-’
cm- ’
4.8
8.6 5.1 8.2 14.8 15.5
1647.0 1642.3 1639.3
* . . HO (hexane-solvent),
0.0 2.4
4.5 8.2
1646.2 1644.1 1643.0 1644.6 1641.2 1642.5 1643.3 1641.0
93-102
solvents AN
1646.8
I2 (1996)
AN
vc=o
8.2 8.6
6.2 3.1
8.2
18.9 14.8
6.5 5.6 5.6
15.5 20.4
6.3 6.9
54
6.3
29.1 23.1 19.3
3.5 7.3 10.3
33.5 37.1 41.3
3.8 4.6 4.8
* * * HO (hexane-solvent),
cm- ’
R.A. Nyquisr Table 3 Infrared data for phenyl
salicylate
Solvent
vC=O cm-’
Hexane Diethyl ether Methyl t-butyl ether Toluene Benzene Carbon tetrachloride Carbon disulfide 1,2-Dichlorobenzene Acetonitrile Nitrobenzene Benzonitrile Methylene chloride Nitromethane t-Butyl alcohol Chloroform Dimethyl sulfoxide Isopropyl alcohol Ethyl alcohol Methyl alcohol
1697.9 1695.7 1696.3 1694.0 1693.4 1695.4 1693.0 1692.1 1694.5 1691.0 1689.8 1692.1 1693.2 1696.1 1690.7 1689.1 1695.5 1694.7
1 wt./vol.% * * . HO,
et al. / Vibrational
in various AN
Specrroscopy
vC=O(HO)~, cm-’
vC=O * * - HO (hexane-solvent), cm-’
vc=o * * - HOvC=O(HO&, cm-’
0.0 2.3 1.6 3.9 4.5 2.5 4.9 5.8 3.4 7.0 2.2 5.8 4.7 1.9 7.3 8.8 2.5
8.2 8.6 8.2 18.9 14.8 15.5 20.4
Fig. 1. Plots of vC=O * * * HO for 2-hydroxyacetophenone, for each of the solvents used in this study.
95
solvents
0.0 3.9
29.1 23.1 19.3 33.5 37.1 41.3
12 (19961 93-102
1680.30 1680.94
methyl
salicylate,
17.6 17.0
and phenyl
salicylate
vs. the solvent
acceptor
number
(AN)
96
R.A. Nyquist
et al./
Vibrational
Spectroscopy
the 1 g/100 ml carbonyl compound in CHCl, solution (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1 ml, sequentially) using a calibrated syringe so that 1 g/100 ml samples of the carbonyl compound in CHCl,/CCl, solutions were obtained ranging from 10.74 to 54.62 mole% CHCl,/CCl,. Similarly, higher mole% CHCl,/CCl, solutions containing a 1 g/100 ml carbonyl compound were prepared by adding the appropriate volume of the 1 g/100 ml carbonyl compound Ccl, solution to the 1 ml of the 1 g/100 ml carbonyl compound CHCl, solution. The nineteen samples were stoppered and shaken to ensure uniform solutions. These solutions were placed in 0.5 mm ISBr cells. Each sample was scanned 64 times and the spectral resolution obtained was 4 cm-‘. The frequency data reported in this study were those printed out by the computer using the peak-picking program. The data are reported to two significant figures, since these frequencies corre-
12 (1996)
93-102
late in a smooth manner with mole% CHCl ,/Ccl,.
the change in the
3. Results and discussion Tables l-3 list IR data for 2-hydroxyacetophenone, methyl salicylate and phenyl salicylate in various solvents, respectively. These three compounds are all intramolecularly hydrogen bonded as illustrated in Fig. 1. Fig. 1 shows plots of the uC=O . . . HO vs. the solvent acceptor number AN. These plots show that the AN values of the four alcohols (solvents nos. 14, 17-19) vs. uc=o * * * HO for 2-hydroxyacetophenone and methyl salicylate correlate in a linear manner. However, they do not correlate together with the plots of uc=o * . . HO vs. AN for the other solvents. In the case of phenyl salicylate, the plot of
C
18 -
l l*
17 --
$9 \
16 --
A) 2-Hydmxy Acetophenone B) Methyl Sakybte C) Phenyl Salkylate
15 -14 --
uC=O:HO (Hexane) *vc-0:HO 60hll0
13
--
,*
--
”
A
B
10 -‘\
’ 16 9. 14
9 --
Fig. 2. Plots of vC=O v=o * * * HO(hexane)
- * * HO for 2-hydroxyacetophenone, methyl salicylate, and phenyl and vC=O * * * HOkolvent) for each of the other solvents.
salicylate
vs. the frequency
difference
between
R.A. Nyquist Table 4 A comparison of the vC=O* frequencies in various solvents
* . HO
et al./ Vibrational
frequencies
Solvent
A
B
ccl, (CH,),SO
2-Hydroxyacetophenone 1646.2 1639.3
Acetophenone 1691 1682
Methyl salicylate 1682.5
Methyl 1730
benzoate
ccl,
Phenyl 1743 1734 1733
benzoate
ccl, CHCl, (CH,),SO
Phenyl salicylate 1695.4 1690.7 1689.1
vs. vC=O 03) - (A)
(8) 44.8 42.7
47.5 (8) 47.6 43.3 43.9
vc=o *
* * HO vs. AN for the three alcohols (solvents nos. 14, 17, 18) also correlate in a linear manner. Only one vC=O * * * HO band was observed for phenyl salicylate in methyl alcohol (solvent no. 19). In ethyl alcohol (solvent no. 18), two vC=O * * * HO bands were observed for phenyl salicylate, and the lower frequency band occurs at a frequency comparable to that for vC=O * * * HO in CH,OH. Extrapolation of the plot of vC=O . * * HO vs. AN Table 5 Infrared data for 2-hydroxyacetophenone solution Mole% 0.00 10.74 19.40 26.53 32.50 37.57 41.93 45.73 49.06 52.00 54.62 51.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00
CHCl,
/Ccl,
vc=o 1646.2 1645.6 1645.2 1644.9 1644.6 1644.5 1644.2 1644.0 1643.9 1643.8 1643.6 1643.5 1643.4 1643.3 1643.1 1643.0 1642.9 1642.8 1642.7 1642.6 1642.3
in CHCl, * * *HO,
and/or cm-’
Ccl,
Spectroscopy
I2 11996) 93-102
Table 6 Infrared data for methyl tions Mole%
CHCl,
/Ccl,
0.00 5.37 10.74 19.40 26.53 32.50 37.57 41.93 45.73 49.06 52.00 54.62 57.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00
0.00 10.74 19.40 26.53 32.50 37.57 41.93 45.73 49.06 52.00 54.62 57.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00
salicylate
vC=O
in CHCl,
* * * HO, cm-’
1682.5 1682.2 1682.0 1681.7 1681.5 1681.3 1681.1 1681.0 1680.8 1680.6 1680.5 1680.2 1680.3 1680.1 1680.0 1679.8 1679.6 1679.5 1679.2 1678.9 1678.7 1678.2
Table 7 Infrared data for phenyl tions Mole%
97
CHCI,
/Ccl,
and/or A(vC=O
Ccl,
solu-
* * aHO)
0.150 0.498 0.477 0.472 0.463 0.457 0.452 0.448 0.445 0.441 0.437 0.436 0.435 0.432 0.429 0.430 0.427 0.426 0.426 0.421 0.416 0.410
salicylate
in CHCl,
vc=o 1695.4 1695.0 1694.7 1694.3 1694.0 1693.7 1693.3 1693.1 1693.0 1692.7 1692.6 1692.5 1692.2 1692.1 1691.9 1691.7 1691.5 1691.3 1691.1 1690.9 1690.6
and/or
* * *HO,
cm-’
Ccl,
solu-
98
R.A. Nyquist
et al. / Vibrational
for the solvents (nos. 2, 5, 6, 8, 10, 11, 16-19) for phenyl salicylate shows that there is a reasonably linear correlation. Thus, in the case of phenyl salicylate in methyl and ethyl alcohol the lower frequency band actually results from a uC=O mode that is both intra- and intermolecularly hydrogen bonded which we designate uC=O...(HO),. We did not observe K=O...(HO), for phenyl salicylate in solution with isopropyl or tert-butyl alcohol (solvents nos. 17 and 14). These data again show that the AN values for the alkyl alcohols include the energy required for intermolecular hydrogen bonding. The AN values for the alcohols surrounding the solute molecules, but not forming intramolecular hydrogen bonds with the C=O group, can be estimatedby extrapolation of the solvent points 14, 17-19 onto the plots including solvents 1 and 16 with subsequentextrapulation to the AN axis. Fig. 1 also showsthat all three compoundsexhibit similar correlation in solution with solvents 9 (acetonitrile), 12 (methylene chloride), and 15 (chlo-
Spectroscopy
93-102
reform), and these plots occur at higher frequency than me three plots that include solvents 1 (hexane) and 16 (dimethyl sulfoxide). These three solvents contain relatively acidic CH,, CH,, or CH protons which can intermolecularly hydrogen bond with a C=O group. This suggeststo us that their AN values also include energy for intermolecular hydrogen bonding. In the present cases,the carbonyl group is already intramolecularly hydrogen bonded, and steric factors of the OH group also prevent normal solutesovent interaction with the carbonyl group. Thus, the intramolecular hydrogen bond occupiesone available bonding site of the carbonyl group which sterically prevents an intermolecular interaction or intermolecular hydrogen bond between the solute and solvent at the site of intramolecular hydrogen bond with the carbonyl group. The basicity of the carbonyl group is less when intramolecularly hydrogen bonded, and this also alters the strength of, for example, an intermolecular hydrogen bond formed between the solvent and solute.
2.Hydroxy
Acetophemme
1) C-O...HO,
Fig. 3. A plot of vC=O
12 (1996)
* - * HO for 2-hydroxyacetophenone
cm-’
vs. the mole 8 CHCl,/CCl,.
R.A. Nyquist
et al. / Vibrational
Spectroscopy
Fig. 2 shows plots of the vC=O * * * HO vs. the frequency difference between vC=O . * * HO in hexane and vC=O * . * HO (solvent). This type of plot is always linear, and it has no theoretical chemical significance. They do serve to show that the points on the three plots are not in the same numerical sequence. Steric differences between the solute and solvents prevent the exact solute-solvent interaction in each case, and this we believe is why the vC=O frequency shift is not the same for each of the compounds in the same solvent. Table 4 compares the vC=O * s * HO frequencies for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate vs. the vC=O frequencies for acetophenone, methyl benzoate, and phenyl benzoate in different solvents, respectively. These comparisons show that the intramolecularly hydrogen bonded vc=o * * * HO group lowers the VC =0 bond frequency in the range 42.7-47.6 cm-‘. Part of the frequency decrease is the result of intramolecular hydrogen bonding. The chemical effect (induction
12 (1996)
. * * HO for methyl
99
and resonance) of the hydroxyl group would also contribute toward lowering of the vC=O frequency. The field effect of the oxygen atom of the hydroxyl group upon the carbonyl group raises the vC=O frequency. The overall effect is that hydrogen bonding and the induction and resonance contributions overrides any contribution from the field effect. Tables 5-7 list IR data for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate in CHClJCCl, solutions, respectively. In going from Ccl, solution to CHCl, solution, the vc=o * . * HO frequency for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate decreases by 3.8, 4.3, and 4.8 cm-‘, respectively. In going from Ccl, solution to solution in (CH,),SO, the vC=O * . * HO frequency for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate decreases by 6.9, 7.2, and 6.3 cm-‘, respectively. In the case of phenyl benzoate, the VC = 0 frequency decreases in frequency by 9 cm-’ in going from solution in Ccl, to solution in CHCl,,
H / ‘.. 0 0 II 0 0’ 6-
Fig. 4. A plot of vC=O
93-102
salicylate
a3
vs. mole 5% CHCl,/CCl,.
100
R.A. Nyquist
et al. / Vibrational
Spectroscopy
and by 10 cm-’ in going from solution in Ccl, to solution in (CH,),SO [8]. In the case of acetophenone, vC=O decreases 9 cm-’ in going from solution in Ccl, to solution in (CH,),SO [8]. The uc=o * * * HO frequency decreases are less than those for uC=O in going from solution in CHCl, or (CH,),SO, and this we attribute to the fact that in the former case the C=O group is intramolecularly hydrogen bonded, and the OH oxygen atom also is a steric factor in solute-solvent interaction. In addition the C=O - * * HO bond makes the C=O group relative less basic due to the intramolecular hydrogen bonding. These factors effect solute-solvent interaction, and accounts for the observed vC=O * * * HO and uC=O shifts in different solvents. Figs. 3-5 show plots of uC=O * . . HO for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate vs. mole% CHCl,/CCl,, respectively. Fig. 3 shows two linear segments, and these are between 0 and ca. 65 mole% CHClJCCl, and between ca. 64 and 100 mole% CHClJCCl,. Fig. 4
12 (1996)
93-102
shows three linear segments. These are between 0 and 10.7 mole% CHClJCCl,, 10.7 and 45.7 mole% CHClJCCl,, and between 45.7 and 100 mole% CHClJCCl,. Fig. 5 shows three linear segments. These are between 0 and 23 mole% CHCl,/CCl,, 23 and 68.7 mole% CHCl,/CCl,, and between 68.7 and 100 mole% CHClJCCl,. These data show that as the mole% CHCl,/CCl, is increased different CHCl,/CCl, complexes are formed with the solute, and these complexex cause vC=O * * * HO to decrease in frequency. Table 8 lists a comparison for the mole ratio CCl,/CHCl,/solute for each linear segment for each plot of the mole% CHCl,/CCl, vs. uc=o * * . HO for 2-hydroxyacetophenone, methyl salicylate and phenyl slicylate, respectively. In the case of 2-hydroxyacetophenone, one linear segment is in the range 12.4 to 9.7 Ccl,/0 to 15 CHCl,/l mole solute, and the other linear segment is in the range 9.7 to 0 Ccl,/15 CHCl,/l mole solute. For methyl salicylate, one linear segment is in the range 15.8 to 2 Ccl,/0 to 19 CHCl,/l mole
H / ‘.. 0 0 I x) 0 0 0 6^
v C=O...HO, Fig. 5. A plot of vC=O
- - * HO for phenyl
salicylate
cm’
vs. the mole ‘% CHCl,/CCl,.
R.A. Nyquisf
et al./ Vibrational
X - CH),
Scheme
OCH3,
Specfroscopy
and
1. Suggested
solute, another linear segment is in the range 2 to 8.7 Ccl,/19 CHCl,/l mole solute, and the other linear segment is in the range 8.7 to 0 Ccl,/ 19 CHCl,/l mole solute. For phenyl salicylate, one linear segment is in the range 22.2 to 6.1 CClJ26.7 CHClJl mole solute, an other linear segment is in the range 6.1 to 18.4 CClJ26.7 CHCl,/l mole solute, and the other linear segment is in the range 18.4 to 0 CClJ26.7 CHCl,/l mole solute. Suggested type
0%
12 (1996)
* - * HO for 1 wt. % solutions from 0 to 100.
of methyl
101
H5
type complexes
complexes are in Scheme 1 (X = CH,, OCH,,orOC,H,). As n is decreased, y would increase, and since Cl of Ccl, is more basic than Cl of CHCl,, it would be expected that the Cl atom of Ccl, would preferentially complex with the carbonyl atom. An intermolecular hydrogen bonding equilibrium would also exist between Ccl, and CHCl, at all but the 0 and 100 mole% CHClJCCl, concentrations.
Y C-O...HO,
Fig. 6. A plot of vC=O CHCI,/CCl, is changed
93-102
salicylate
cm’
vs. the absorbance
of vC=O
* * * HO
as the mole %
102
R.A. Nyquist
/Ccl,
0 ca. 65 100 0 10.7 45.7 100 0 23 68.7 100
Mole ratio CHCl, /CHCl, solute 12.4/O/ 9.7/15/l o/15/1 158/O/l 2/19/l 8.7/19/l o/19/1 22.2/o/ 6.1/26.7/l 18.4/26.7/l O/26.7/
in the linear segments and the mole
of the ratio
Solute /
1
1
1
Spectroscopy
12 (1996)
93-102
uc=o *
Table 8 A comparison of the break points plots vs. mole’% CHCI,/CCI, Ccl, /CHCI, /solute Mole% CHCl,
et al. / Vibrational
2-hydroxyacetophenone 2-hydroxyacetophenone 2-hydroxyacetophenone methyl salicylate methyl salicylate methyl salicylate methyl salicylate phenyl salicylate phenyl salicylate phenyl salicylate phenyl salicylate
Fig. 6 shows a plot of the uC=O + * * HO vs. the absorbance for A(&=0 * . . HO) for each of the solutions from 0 to 100 mole% CHCl,/CCl, solutions for methyl salicylate. Since the concentration of the methyl salicylate is within experimental error constant for each of the solutions, the uc=o * * e HO absorbance must increase as the mole% CHCls/CCl, is increased, but it does not increase in a linear manner.
4. Conclusions The intramolecularly hydrogen bonded 2-hydroxy compounds exhibit vC=O * * * HO at lower frequency than vC=O for similar compounds which are not intramolecularly hydrogen bonded. Hydrogen bonding is one reason the vC=O frequency is lowered. Another reason is that the inductive and resonance contributions of the OH group through the phenyl group to the carbonyl group weakens the C=O bond, and hence lowers the frequency. Apparently, the above effects more than off-set the field effect of the OH oxygen atom which raises the vC=O frequency. The 2-hydroxy group sterically alters the spatial distance between the sites solutesolvent interaction, as well as decreases the overall basicity of the carbonyl group. Therefore, the
* * HO vibration does not shift as much to lower frequency in going from solution in CCl, to solution in CHCl,, or (CH,),SO. Steric factors between solute and solvent is the reason the AN values for the solvents cannot be used to predict exact uC=O frequencies in different solvents.
References [II R.A. P.1 R.A.
Nyquist, Appl. Spectrosc.. 40 (1986) 79. Nyquist. Appl. Spectrosc.. 40 (1986) 336. 131 R.A. Nyquist, V. Chrzan and J. Houck, Appl. Spectrosc., 43 (1989) 981. [41 R.A. Nyquist, C.L. Putzig and D.L. Hasha, Appl. Spectrosc., 43 (1989) 1049. 151 R.A. Nyquist, T.M. Kirchner and H.A. Fouchea, Appl. Spectrosc., 43 (1989) 1053. 161R.A. Nyquist, Appl. Spectrosc., 43 (1989) 1208. 171 R.A. Nyquist, Appl. Spectrosc., 43 (1989) 1374. [81 R.A. Nyquist, V. Chrzan, T.M. Kirchner, L. Yurga and CL. Putzig, Appl. Spectrosc., 44 (1990) 243. [91 R.A. Nyquist, Appt. Spectrosc., 44 (1990) 425. [lOI R.A. Nyquist, Appl. Spectrosc., 44 (1990) 433. [Ill R.A. Nyquist, Appl. Spectrosc.. 44 (1990) 438. [I21 R.A. Nyquist. Appl. Spectrosc.. 44 (1990) 783. [I31 R.A. Nyquist and S.E. Settineri, Appl. Spectrosc., 44 (1990) 791. [141 R.A. Nyquist. Appl. Spectrosc., 44 (1990) 1405. 1151 R.A. Nyquist and SE. Settineri. Appl. Spectrosc., 44 (1990) 1552. [t61 R.A. Nyquist and S.E. Settineri, Appl. Spectrosc., 44 (1990) 1629. [171 R.A. Nyquist, Appl. Spectrosc., 45 (1991) 92. iI81 R.A. Nyquist, H.A. Fouchea, G.A. Hoffman and D.L. Hasha, Appl. Spectrosc., 45 (1991) 860. 1191 R.A. Nyquist and S.E. Settineri, Appl. Spectrosc., 45 (1991) 1075. [201R.A. Nyquist, D.A. Luoma and D.W. Wilkening, Vib. Spectrosc., 2 (1991) 61. 1213R.A. Nyquist and D.A. Luoma. Appl. Spectrosc., 45 (1991) 1491. I221R.A. Nyquist and D.A. Luoma, Appl. Spectrosc., 45 (1991) 1497. [231R.A. Nyquist, Appl. Spectrosc., 46 (1992) 306. ]241 R.A. Nyquist, S.E. Settineri and D.A. Luoma, Appl. Spectrosc., 45 (1992) 293. [25] R.A. Nyquist and CL. Putzig, Vib. Spectrosc., 3 (1992) 35. [26] R.A. Nyquist, Structural information from infrared studies on solute-solvent interactions, PhD. Thesis, Utrecht University. 1994. [27] V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York. 1978, p. 29.
Vibrational
Spectroscopy
12 ( 1996) 93- 102
Infrared study of intramolecularly hydrogen bonded aromatic carbonyl containing compounds in various solvents R.A. Nyquist *, C.L. Putzig, T.L. Clark, A.T. McDonald Dow Chemical
Company,
Analytical Received
Sciences Laboratory, 30 May
1995; accepted
I897F
Building,
23 December
Midland,
MI 48667,
USA
1995
Abstract The intramolecularly hydrogen bonded OH group sterically alters the spatial distance between the sites of solute-solvent interaction, and the larger spatial distance causes a weaker solute-sovent interaction. Intramolecular hydrogen bonding also causes the carbonyl group to be relatively less basic which decreases the strength of an intermolecular hydrogen bond formed between the solute and the acidic proton of the solvent. Therefore, there is a smaller vC=O . * * HO frequency decrease compared to vC=O for similar compounds not intramolecularly hydrogen bonded in going from solution in Ccl, to solution in CHCl,, or (CH,),SO. Keywords;
Intramolecularly
H-bonded
aromatic
carbonyl
containing
1. Introduction In this laboratory, we have obtained structural information from IR and Raman studies on solutesolvent interactions [l-26]. In our present study of intramolecularly hydrogen bonded carbonyl containing compounds, we were interested in determining how the OH group affects interaction of the C=O group with different solvents. The solvent acceptor number (AN) was developed by Gutmann, and AN is reported to be a measureof solvent electrophilicity. The AN value is defined as a dimensionlessnumber related to a relative chemical shift of 31P in (C,H,),P=O in that particular solvent, with hexane as a reference solvent on one hand, and
* Corresponding 0924.2031/96/$15.00 PII SO924-203
author. Copyright 1(96)00005-7
0 1996 Elsevier
Science
compounds
(C,H,),P=O . * SbCl, in 1,2-dichloroethaneon the other, to which the acceptor numbers0 and 100 have been assigned[27]. The AN values of a solvent are thought to be useful in predicting vibrational group frequency data of chemicals in a particular solvent, and this study tests the validity of this concept.
2. Experimental Infrared spectra were recorded with the Nicolet 710 FT-IR system which is purged with dry nitrogen in order to exclude ambient air from the sample compartment. The compounds used in this study were purchased from Aldrich or Merck, and were studied without further purification. Carbonyl compounds were prepared as 1 g/100 ml in Ccl, solution and in CHCl, solution. Mole% solutions of CHC13/CCl, were preparedusing stock solutions of
B.V. All rights reserved
94
R.A. Nyquist
Table 1 Infrared data for 2-hydroxyacetophenone
et al. / Vibrational
1 wt./vol.%
Solvent
iC=O...HO,
Hexane Diethyl ether Methyl t-butyl ether Toluene Benzene Carbon tetrachloride Carbon disulfide 1,2-Dichlorobenzene Acetonitrile Nitrobenzene Benzonitrile Methylene chloride Nitromethane t-Butyl alcohol Chloroform Dimethyl sulfoxide Isopropyl alcohol Ethyl alcohol Methyl alcohol
1649.2
in various
cm-i
Spectroscopy
vc=o
0.0 3.9 2.5
1646.7 1644.7 1644.4
1646.2 1645.5 1645.1
3.0
18.9
4.5 8.0 6.7
6.2
20.4 8.2 29.1
5.9
23.1 19.3 33.5
6.9 9.9 3.0
37.1 41.3
3.7 4.1
2.2
each carbonyl compound so that within experimental error the concentration of the solute is constant in each of the mole% CHCl,/CCl, solutions. For example, 1 ml samples of the 1 g/100 ml Ccl, stock solutions were placed separately into ten glass botTable 2 Infrared data
for methyl
salicylate
1 wt./vol.%
in various
tles and 1 ml samples of the 1 g/100 ml CHCl, stock solution were placed separetely into nine glass bottles using calibrated micro syringes. Then, to each of the 1 ml 1 g/100 ml carbonyl compound in Ccl, solution samples we added a calibrated volume of
solvents
Solvent
vc=o
Hexane Dimethyl ether Methyl t-butyl ether Toluene Benzene Carbon tetrachloride Carbon disulfide 1,2-Dichlorobenzene Acetonitrile Nitrobenzene Benzonitrile Methylene chloride Nitromethane t-Butyl alcohol Chloroform Dimethyl sulfoxide Isopropyl alcohol Ethyl alcohol Methyl alcohol
1685.6
0.0
0.0
1682.3
3.9
3.3
1682.2 1679.9 1679.4 1682.5
3.4 5.7
1680.4
1679.2 1680.1 1680.1 1679.4 1678.7 1679.3 1682.1 1678.3 1675.3
1681.8 1681.1 1680.2
* * * HO, cm-’
cm- ’
4.8
8.6 5.1 8.2 14.8 15.5
1647.0 1642.3 1639.3
* . . HO (hexane-solvent),
0.0 2.4
4.5 8.2
1646.2 1644.1 1643.0 1644.6 1641.2 1642.5 1643.3 1641.0
93-102
solvents AN
1646.8
I2 (1996)
AN
vc=o
8.2 8.6
6.2 3.1
8.2
18.9 14.8
6.5 5.6 5.6
15.5 20.4
6.3 6.9
54
6.3
29.1 23.1 19.3
3.5 7.3 10.3
33.5 37.1 41.3
3.8 4.6 4.8
* * * HO (hexane-solvent),
cm- ’
R.A. Nyquisr Table 3 Infrared data for phenyl
salicylate
Solvent
vC=O cm-’
Hexane Diethyl ether Methyl t-butyl ether Toluene Benzene Carbon tetrachloride Carbon disulfide 1,2-Dichlorobenzene Acetonitrile Nitrobenzene Benzonitrile Methylene chloride Nitromethane t-Butyl alcohol Chloroform Dimethyl sulfoxide Isopropyl alcohol Ethyl alcohol Methyl alcohol
1697.9 1695.7 1696.3 1694.0 1693.4 1695.4 1693.0 1692.1 1694.5 1691.0 1689.8 1692.1 1693.2 1696.1 1690.7 1689.1 1695.5 1694.7
1 wt./vol.% * * . HO,
et al. / Vibrational
in various AN
Specrroscopy
vC=O(HO)~, cm-’
vC=O * * - HO (hexane-solvent), cm-’
vc=o * * - HOvC=O(HO&, cm-’
0.0 2.3 1.6 3.9 4.5 2.5 4.9 5.8 3.4 7.0 2.2 5.8 4.7 1.9 7.3 8.8 2.5
8.2 8.6 8.2 18.9 14.8 15.5 20.4
Fig. 1. Plots of vC=O * * * HO for 2-hydroxyacetophenone, for each of the solvents used in this study.
95
solvents
0.0 3.9
29.1 23.1 19.3 33.5 37.1 41.3
12 (19961 93-102
1680.30 1680.94
methyl
salicylate,
17.6 17.0
and phenyl
salicylate
vs. the solvent
acceptor
number
(AN)
96
R.A. Nyquist
et al./
Vibrational
Spectroscopy
the 1 g/100 ml carbonyl compound in CHCl, solution (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1 ml, sequentially) using a calibrated syringe so that 1 g/100 ml samples of the carbonyl compound in CHCl,/CCl, solutions were obtained ranging from 10.74 to 54.62 mole% CHCl,/CCl,. Similarly, higher mole% CHCl,/CCl, solutions containing a 1 g/100 ml carbonyl compound were prepared by adding the appropriate volume of the 1 g/100 ml carbonyl compound Ccl, solution to the 1 ml of the 1 g/100 ml carbonyl compound CHCl, solution. The nineteen samples were stoppered and shaken to ensure uniform solutions. These solutions were placed in 0.5 mm ISBr cells. Each sample was scanned 64 times and the spectral resolution obtained was 4 cm-‘. The frequency data reported in this study were those printed out by the computer using the peak-picking program. The data are reported to two significant figures, since these frequencies corre-
12 (1996)
93-102
late in a smooth manner with mole% CHCl ,/Ccl,.
the change in the
3. Results and discussion Tables l-3 list IR data for 2-hydroxyacetophenone, methyl salicylate and phenyl salicylate in various solvents, respectively. These three compounds are all intramolecularly hydrogen bonded as illustrated in Fig. 1. Fig. 1 shows plots of the uC=O . . . HO vs. the solvent acceptor number AN. These plots show that the AN values of the four alcohols (solvents nos. 14, 17-19) vs. uc=o * * * HO for 2-hydroxyacetophenone and methyl salicylate correlate in a linear manner. However, they do not correlate together with the plots of uc=o * . . HO vs. AN for the other solvents. In the case of phenyl salicylate, the plot of
C
18 -
l l*
17 --
$9 \
16 --
A) 2-Hydmxy Acetophenone B) Methyl Sakybte C) Phenyl Salkylate
15 -14 --
uC=O:HO (Hexane) *vc-0:HO 60hll0
13
--
,*
--
”
A
B
10 -‘\
’ 16 9. 14
9 --
Fig. 2. Plots of vC=O v=o * * * HO(hexane)
- * * HO for 2-hydroxyacetophenone, methyl salicylate, and phenyl and vC=O * * * HOkolvent) for each of the other solvents.
salicylate
vs. the frequency
difference
between
R.A. Nyquist Table 4 A comparison of the vC=O* frequencies in various solvents
* . HO
et al./ Vibrational
frequencies
Solvent
A
B
ccl, (CH,),SO
2-Hydroxyacetophenone 1646.2 1639.3
Acetophenone 1691 1682
Methyl salicylate 1682.5
Methyl 1730
benzoate
ccl,
Phenyl 1743 1734 1733
benzoate
ccl, CHCl, (CH,),SO
Phenyl salicylate 1695.4 1690.7 1689.1
vs. vC=O 03) - (A)
(8) 44.8 42.7
47.5 (8) 47.6 43.3 43.9
vc=o *
* * HO vs. AN for the three alcohols (solvents nos. 14, 17, 18) also correlate in a linear manner. Only one vC=O * * * HO band was observed for phenyl salicylate in methyl alcohol (solvent no. 19). In ethyl alcohol (solvent no. 18), two vC=O * * * HO bands were observed for phenyl salicylate, and the lower frequency band occurs at a frequency comparable to that for vC=O * * * HO in CH,OH. Extrapolation of the plot of vC=O . * * HO vs. AN Table 5 Infrared data for 2-hydroxyacetophenone solution Mole% 0.00 10.74 19.40 26.53 32.50 37.57 41.93 45.73 49.06 52.00 54.62 51.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00
CHCl,
/Ccl,
vc=o 1646.2 1645.6 1645.2 1644.9 1644.6 1644.5 1644.2 1644.0 1643.9 1643.8 1643.6 1643.5 1643.4 1643.3 1643.1 1643.0 1642.9 1642.8 1642.7 1642.6 1642.3
in CHCl, * * *HO,
and/or cm-’
Ccl,
Spectroscopy
I2 11996) 93-102
Table 6 Infrared data for methyl tions Mole%
CHCl,
/Ccl,
0.00 5.37 10.74 19.40 26.53 32.50 37.57 41.93 45.73 49.06 52.00 54.62 57.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00
0.00 10.74 19.40 26.53 32.50 37.57 41.93 45.73 49.06 52.00 54.62 57.22 60.07 63.28 66.73 70.65 75.06 80.06 85.05 92.33 100.00
salicylate
vC=O
in CHCl,
* * * HO, cm-’
1682.5 1682.2 1682.0 1681.7 1681.5 1681.3 1681.1 1681.0 1680.8 1680.6 1680.5 1680.2 1680.3 1680.1 1680.0 1679.8 1679.6 1679.5 1679.2 1678.9 1678.7 1678.2
Table 7 Infrared data for phenyl tions Mole%
97
CHCI,
/Ccl,
and/or A(vC=O
Ccl,
solu-
* * aHO)
0.150 0.498 0.477 0.472 0.463 0.457 0.452 0.448 0.445 0.441 0.437 0.436 0.435 0.432 0.429 0.430 0.427 0.426 0.426 0.421 0.416 0.410
salicylate
in CHCl,
vc=o 1695.4 1695.0 1694.7 1694.3 1694.0 1693.7 1693.3 1693.1 1693.0 1692.7 1692.6 1692.5 1692.2 1692.1 1691.9 1691.7 1691.5 1691.3 1691.1 1690.9 1690.6
and/or
* * *HO,
cm-’
Ccl,
solu-
98
R.A. Nyquist
et al. / Vibrational
for the solvents (nos. 2, 5, 6, 8, 10, 11, 16-19) for phenyl salicylate shows that there is a reasonably linear correlation. Thus, in the case of phenyl salicylate in methyl and ethyl alcohol the lower frequency band actually results from a uC=O mode that is both intra- and intermolecularly hydrogen bonded which we designate uC=O...(HO),. We did not observe K=O...(HO), for phenyl salicylate in solution with isopropyl or tert-butyl alcohol (solvents nos. 17 and 14). These data again show that the AN values for the alkyl alcohols include the energy required for intermolecular hydrogen bonding. The AN values for the alcohols surrounding the solute molecules, but not forming intramolecular hydrogen bonds with the C=O group, can be estimatedby extrapolation of the solvent points 14, 17-19 onto the plots including solvents 1 and 16 with subsequentextrapulation to the AN axis. Fig. 1 also showsthat all three compoundsexhibit similar correlation in solution with solvents 9 (acetonitrile), 12 (methylene chloride), and 15 (chlo-
Spectroscopy
93-102
reform), and these plots occur at higher frequency than me three plots that include solvents 1 (hexane) and 16 (dimethyl sulfoxide). These three solvents contain relatively acidic CH,, CH,, or CH protons which can intermolecularly hydrogen bond with a C=O group. This suggeststo us that their AN values also include energy for intermolecular hydrogen bonding. In the present cases,the carbonyl group is already intramolecularly hydrogen bonded, and steric factors of the OH group also prevent normal solutesovent interaction with the carbonyl group. Thus, the intramolecular hydrogen bond occupiesone available bonding site of the carbonyl group which sterically prevents an intermolecular interaction or intermolecular hydrogen bond between the solute and solvent at the site of intramolecular hydrogen bond with the carbonyl group. The basicity of the carbonyl group is less when intramolecularly hydrogen bonded, and this also alters the strength of, for example, an intermolecular hydrogen bond formed between the solvent and solute.
2.Hydroxy
Acetophemme
1) C-O...HO,
Fig. 3. A plot of vC=O
12 (1996)
* - * HO for 2-hydroxyacetophenone
cm-’
vs. the mole 8 CHCl,/CCl,.
R.A. Nyquist
et al. / Vibrational
Spectroscopy
Fig. 2 shows plots of the vC=O * * * HO vs. the frequency difference between vC=O . * * HO in hexane and vC=O * . * HO (solvent). This type of plot is always linear, and it has no theoretical chemical significance. They do serve to show that the points on the three plots are not in the same numerical sequence. Steric differences between the solute and solvents prevent the exact solute-solvent interaction in each case, and this we believe is why the vC=O frequency shift is not the same for each of the compounds in the same solvent. Table 4 compares the vC=O * s * HO frequencies for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate vs. the vC=O frequencies for acetophenone, methyl benzoate, and phenyl benzoate in different solvents, respectively. These comparisons show that the intramolecularly hydrogen bonded vc=o * * * HO group lowers the VC =0 bond frequency in the range 42.7-47.6 cm-‘. Part of the frequency decrease is the result of intramolecular hydrogen bonding. The chemical effect (induction
12 (1996)
. * * HO for methyl
99
and resonance) of the hydroxyl group would also contribute toward lowering of the vC=O frequency. The field effect of the oxygen atom of the hydroxyl group upon the carbonyl group raises the vC=O frequency. The overall effect is that hydrogen bonding and the induction and resonance contributions overrides any contribution from the field effect. Tables 5-7 list IR data for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate in CHClJCCl, solutions, respectively. In going from Ccl, solution to CHCl, solution, the vc=o * . * HO frequency for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate decreases by 3.8, 4.3, and 4.8 cm-‘, respectively. In going from Ccl, solution to solution in (CH,),SO, the vC=O * . * HO frequency for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate decreases by 6.9, 7.2, and 6.3 cm-‘, respectively. In the case of phenyl benzoate, the VC = 0 frequency decreases in frequency by 9 cm-’ in going from solution in Ccl, to solution in CHCl,,
H / ‘.. 0 0 II 0 0’ 6-
Fig. 4. A plot of vC=O
93-102
salicylate
a3
vs. mole 5% CHCl,/CCl,.
100
R.A. Nyquist
et al. / Vibrational
Spectroscopy
and by 10 cm-’ in going from solution in Ccl, to solution in (CH,),SO [8]. In the case of acetophenone, vC=O decreases 9 cm-’ in going from solution in Ccl, to solution in (CH,),SO [8]. The uc=o * * * HO frequency decreases are less than those for uC=O in going from solution in CHCl, or (CH,),SO, and this we attribute to the fact that in the former case the C=O group is intramolecularly hydrogen bonded, and the OH oxygen atom also is a steric factor in solute-solvent interaction. In addition the C=O - * * HO bond makes the C=O group relative less basic due to the intramolecular hydrogen bonding. These factors effect solute-solvent interaction, and accounts for the observed vC=O * * * HO and uC=O shifts in different solvents. Figs. 3-5 show plots of uC=O * . . HO for 2-hydroxyacetophenone, methyl salicylate, and phenyl salicylate vs. mole% CHCl,/CCl,, respectively. Fig. 3 shows two linear segments, and these are between 0 and ca. 65 mole% CHClJCCl, and between ca. 64 and 100 mole% CHClJCCl,. Fig. 4
12 (1996)
93-102
shows three linear segments. These are between 0 and 10.7 mole% CHClJCCl,, 10.7 and 45.7 mole% CHClJCCl,, and between 45.7 and 100 mole% CHClJCCl,. Fig. 5 shows three linear segments. These are between 0 and 23 mole% CHCl,/CCl,, 23 and 68.7 mole% CHCl,/CCl,, and between 68.7 and 100 mole% CHClJCCl,. These data show that as the mole% CHCl,/CCl, is increased different CHCl,/CCl, complexes are formed with the solute, and these complexex cause vC=O * * * HO to decrease in frequency. Table 8 lists a comparison for the mole ratio CCl,/CHCl,/solute for each linear segment for each plot of the mole% CHCl,/CCl, vs. uc=o * * . HO for 2-hydroxyacetophenone, methyl salicylate and phenyl slicylate, respectively. In the case of 2-hydroxyacetophenone, one linear segment is in the range 12.4 to 9.7 Ccl,/0 to 15 CHCl,/l mole solute, and the other linear segment is in the range 9.7 to 0 Ccl,/15 CHCl,/l mole solute. For methyl salicylate, one linear segment is in the range 15.8 to 2 Ccl,/0 to 19 CHCl,/l mole
H / ‘.. 0 0 I x) 0 0 0 6^
v C=O...HO, Fig. 5. A plot of vC=O
- - * HO for phenyl
salicylate
cm’
vs. the mole ‘% CHCl,/CCl,.
R.A. Nyquisf
et al./ Vibrational
X - CH),
Scheme
OCH3,
Specfroscopy
and
1. Suggested
solute, another linear segment is in the range 2 to 8.7 Ccl,/19 CHCl,/l mole solute, and the other linear segment is in the range 8.7 to 0 Ccl,/ 19 CHCl,/l mole solute. For phenyl salicylate, one linear segment is in the range 22.2 to 6.1 CClJ26.7 CHClJl mole solute, an other linear segment is in the range 6.1 to 18.4 CClJ26.7 CHCl,/l mole solute, and the other linear segment is in the range 18.4 to 0 CClJ26.7 CHCl,/l mole solute. Suggested type
0%
12 (1996)
* - * HO for 1 wt. % solutions from 0 to 100.
of methyl
101
H5
type complexes
complexes are in Scheme 1 (X = CH,, OCH,,orOC,H,). As n is decreased, y would increase, and since Cl of Ccl, is more basic than Cl of CHCl,, it would be expected that the Cl atom of Ccl, would preferentially complex with the carbonyl atom. An intermolecular hydrogen bonding equilibrium would also exist between Ccl, and CHCl, at all but the 0 and 100 mole% CHClJCCl, concentrations.
Y C-O...HO,
Fig. 6. A plot of vC=O CHCI,/CCl, is changed
93-102
salicylate
cm’
vs. the absorbance
of vC=O
* * * HO
as the mole %
102
R.A. Nyquist
/Ccl,
0 ca. 65 100 0 10.7 45.7 100 0 23 68.7 100
Mole ratio CHCl, /CHCl, solute 12.4/O/ 9.7/15/l o/15/1 158/O/l 2/19/l 8.7/19/l o/19/1 22.2/o/ 6.1/26.7/l 18.4/26.7/l O/26.7/
in the linear segments and the mole
of the ratio
Solute /
1
1
1
Spectroscopy
12 (1996)
93-102
uc=o *
Table 8 A comparison of the break points plots vs. mole’% CHCI,/CCI, Ccl, /CHCI, /solute Mole% CHCl,
et al. / Vibrational
2-hydroxyacetophenone 2-hydroxyacetophenone 2-hydroxyacetophenone methyl salicylate methyl salicylate methyl salicylate methyl salicylate phenyl salicylate phenyl salicylate phenyl salicylate phenyl salicylate
Fig. 6 shows a plot of the uC=O + * * HO vs. the absorbance for A(&=0 * . . HO) for each of the solutions from 0 to 100 mole% CHCl,/CCl, solutions for methyl salicylate. Since the concentration of the methyl salicylate is within experimental error constant for each of the solutions, the uc=o * * e HO absorbance must increase as the mole% CHCls/CCl, is increased, but it does not increase in a linear manner.
4. Conclusions The intramolecularly hydrogen bonded 2-hydroxy compounds exhibit vC=O * * * HO at lower frequency than vC=O for similar compounds which are not intramolecularly hydrogen bonded. Hydrogen bonding is one reason the vC=O frequency is lowered. Another reason is that the inductive and resonance contributions of the OH group through the phenyl group to the carbonyl group weakens the C=O bond, and hence lowers the frequency. Apparently, the above effects more than off-set the field effect of the OH oxygen atom which raises the vC=O frequency. The 2-hydroxy group sterically alters the spatial distance between the sites solutesolvent interaction, as well as decreases the overall basicity of the carbonyl group. Therefore, the
* * HO vibration does not shift as much to lower frequency in going from solution in CCl, to solution in CHCl,, or (CH,),SO. Steric factors between solute and solvent is the reason the AN values for the solvents cannot be used to predict exact uC=O frequencies in different solvents.
References [II R.A. P.1 R.A.
Nyquist, Appl. Spectrosc.. 40 (1986) 79. Nyquist. Appl. Spectrosc.. 40 (1986) 336. 131 R.A. Nyquist, V. Chrzan and J. Houck, Appl. Spectrosc., 43 (1989) 981. [41 R.A. Nyquist, C.L. Putzig and D.L. Hasha, Appl. Spectrosc., 43 (1989) 1049. 151 R.A. Nyquist, T.M. Kirchner and H.A. Fouchea, Appl. Spectrosc., 43 (1989) 1053. 161R.A. Nyquist, Appl. Spectrosc., 43 (1989) 1208. 171 R.A. Nyquist, Appl. Spectrosc., 43 (1989) 1374. [81 R.A. Nyquist, V. Chrzan, T.M. Kirchner, L. Yurga and CL. Putzig, Appl. Spectrosc., 44 (1990) 243. [91 R.A. Nyquist, Appt. Spectrosc., 44 (1990) 425. [lOI R.A. Nyquist, Appl. Spectrosc., 44 (1990) 433. [Ill R.A. Nyquist, Appl. Spectrosc.. 44 (1990) 438. [I21 R.A. Nyquist. Appl. Spectrosc.. 44 (1990) 783. [I31 R.A. Nyquist and S.E. Settineri, Appl. Spectrosc., 44 (1990) 791. [141 R.A. Nyquist. Appl. Spectrosc., 44 (1990) 1405. 1151 R.A. Nyquist and SE. Settineri. Appl. Spectrosc., 44 (1990) 1552. [t61 R.A. Nyquist and S.E. Settineri, Appl. Spectrosc., 44 (1990) 1629. [171 R.A. Nyquist, Appl. Spectrosc., 45 (1991) 92. iI81 R.A. Nyquist, H.A. Fouchea, G.A. Hoffman and D.L. Hasha, Appl. Spectrosc., 45 (1991) 860. 1191 R.A. Nyquist and S.E. Settineri, Appl. Spectrosc., 45 (1991) 1075. [201R.A. Nyquist, D.A. Luoma and D.W. Wilkening, Vib. Spectrosc., 2 (1991) 61. 1213R.A. Nyquist and D.A. Luoma. Appl. Spectrosc., 45 (1991) 1491. I221R.A. Nyquist and D.A. Luoma, Appl. Spectrosc., 45 (1991) 1497. [231R.A. Nyquist, Appl. Spectrosc., 46 (1992) 306. ]241 R.A. Nyquist, S.E. Settineri and D.A. Luoma, Appl. Spectrosc., 45 (1992) 293. [25] R.A. Nyquist and CL. Putzig, Vib. Spectrosc., 3 (1992) 35. [26] R.A. Nyquist, Structural information from infrared studies on solute-solvent interactions, PhD. Thesis, Utrecht University. 1994. [27] V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions, Plenum Press, New York. 1978, p. 29.