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Correlation between the frequency shift of the stretching vibration of hydrogen bond complexes and the proton affinity
ELSEVIER
Journal of Molecular Liquids 67
(1995)33 -47
CORRELATION BETWEEN THE FREQUENCY SHIFT OF THE STRETCHING VIBRATION OF HYDROGEN BOND COMPLEXES AND THE PROTON AFFINITY The&e Zeegers-Huyskens Department of Chemistry, University of Leuven Celestijnenlaan 2OOF,B-3001 Heverlee Belgium ABSTRACT
The relative frequency shifts of the stretching vibration A-H---B in hydrogen bond complexes are discussed as a function of APA, the difference between the proton affinity of the A- anion and the base. In a relatively small bPA range (f 50 kcal mol-1) the relations can be considered as linear. The slopes and the intercepts of the correlation are ordered according to : N(sp3) > N(sp2) > anilines > 0(sp3) > 0(sp2) > N(sp). In a broad f~ APA domain, the correlation takes an exponential form. The correlations allow to predict the proton affinity of polyfunctional bases where different sites are available for hydrogen bond formation. The correlations suggest that for uracil, the protonation site in the gas phase is not the same as that in the solid state or in low temperature Argon matrix.
INTRODUCTION A student of the laboratory asked me some years ago if it was possible to estimate the proton affinity (PA) of N-CH3-2-pyrimidone which was not determined experimentally.
I answered that the PA can be estimated with a good accuracy from the hydrogen bond parameters [l]. My husband has always been interested to the acid-base behaviour of the hydrogen bonds. I am very happy to dedicate to him this work on the occasion of his retirement. 0167-7322/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-7322 (95) 00864-O
34
The PA of a base B can be extracted from the linear correlation between tbe enthalpies of hydrogen bond formation ( -AHoH& and APA, the difference between the PA’s of the anion A- and that of the base [2]. The error on the experimental determination of AH”HB is usually of the magnitude of & 0.35 kcal mol-l and as a consequence, tbe inaccuracy on the estimation of PA for hydrogen bonds of medium strength, + 3 kcalmoF1 [3]. The PA of a base can also be estimated from the value of the frequency shifts of the “OH stretching vibration. For a very limited number of systems, including phenols complexed with carbonyl bases [3-61, I deduced the following equation AVOH
/cm-l
= 1090 - 1.34 APA /kJ mol-l = 1090 - 5.63 APA lkcal mol-l) (1)
For weak to medium hydrogen bonds (AvOH = 300 cm-l) the experimental error on AvOH = & 5 cm-l. As a consequence, the error on PA (& 0.9 kcal mol-1) is much smaller than that based on the entltalpies. From the experimental frequency shifts of the phenols - N-CH3-2-pyrimidone complexes [3], a value of 212 kcal mol-’ (887 kImoF’) for the PA of the base can be deduced. It must be noticed here that this value refers to the PA of the oxygen atom of the cat-bony1group, the hydrogen bond being formed at this site in carbon tetrachloride solution and the AvOH values refering to the formation of C=O---HO hydrogen bonds. Infrared [7] and X-ray diffraction studies have proven that the N atom is the protonation site in the solid state. This is also likely to be the case in the gas phase, so that the experimental PA of N-CH3-2-pytimidone would probably be different from 212 kcal mol-‘. Owing to the growing interest of the study of the specific interactions in biological molecules, where hydrogen bond acceptor sites are very often present, the relation between the frequency shifts and the PA will be discussed for different hydrogen bond systems. LITERATURE RESULTS AND DISCUSSION Several authors have discussed the relations between the energetics of hydrogen bonds in the gas phase and the PA or APA values [g-16]. Attempts to correlate the hydrogen bond parameters in solution and the PA are much scarcer [17 - 211. Of particular interest is the reduced PA scale defined by Pimentel, widely used in order to predict the proton transfer structure in low temperature matrices [22-231. Owing to the fact that the VOH frequencies depend on the nature of the proton donor, the relative frequency shifts AU/Vwill be considered in this work. Tables 1 - 6 report WV and the APA values for different OH--N and OH--O systems where the N and 0 atoms have different hybridizations. All the data are relative to dilute carbon
35
tetrachloride solution. The frequency of the VO_H___B band is sensitive to the environment and generally increases with the dielectric constant of the medium [24-261. For the water complexes, the decoupled AVOH values have been computed by a previously described method [20] based on the correlation of Luck and .Schi&erg [27-281. It must be pointed out here that in all the complexes considered in this work, the “OH band is relatively broad, sometimes characterized by a fine structure but presents a well-defined maximum. The complexes involving hydroxylic proton donors and the esters are not considered in this work. For the same -AH values, the AVOHvalues are lower than for other carbonyl bases. This can be explained by the fact that the lone pair of electrons of the carbonyl group in cis-position with respect to the alcoxy group is markedly more involved in hydrogen bonding than the tram lone pair. However, as shown by dipole moment studies [30] and by ab initio calculations [31], the angle formed between the C =0 direction and the direction of the hydrogen bond takes values between 40 and 48”, so that the direction of the O-H vibration does not exactly conincide with the direction of the lone pair and smaller AVOH (as compared with ketones and amides) will result. Table 1 Relative frequency shift Au/v and APA/kcal mol-' values for 0-H---N(sp3) systems. Solvent : Carbon tetrachloride Proton
donor
3-HO2 phenol
Base
102Av/, (exp) (calc.eq.2)
APAafb
29.3c
29.0
104.7
28.8d
28.2
106.8
29.ld 28.2d
28.1 27.5
107.3
24.1d
24.3
(C2H5)2NH piperidine
22.7d
24.2
117.9 118.4
23.3d
23.9
119.1
4-Cl phenol Phenol Phenol
(C2H5)2NH piperidine
23.0d 22.7d 22.4d
23.7
119.6
21.9 21.7
125.0 125.5
4-CH3 phenol 4-CH30 phenol
(C2H5)2NH
22.5d
(C2H5)2NH
22.3d
21.4 21.3
126.4 126.7
4-NC2 phenol 4-NC2 phenol 4-CN phenol 3-Cl phenol 3-Cl phenol 4-Cl phenol
(C2H5)3N piperidine (C2H5)2NH (C2H5)2NH piperidine
(C2H5)2NH
108.9
a : The PA of the bases are taken from ref.32 b : The PA of the A- ions are taken from ref.33, taking a PA of 351.4 kcal mol-l for the unsubstituted phenolate ion. c : ref.34 d : ref.35
36 Table 2 Relative frequency shift Aviv and APA/kcal mol-1 values for 0-H---N(sp') systems. Solvent : Carbon tetrachloride Proton donor
Base
102A~/v (exp) (calc.eq.9) APAa#b
Phenols 4-N+
phenol
4-F phenol
Pyridine
19.80
N,N dimethylaminopyrid. 18.00c
17.04
112.7
17.23
115.8 116.5 123.9
3-NC2 phenol
Pyridine
17.20
16.06
3-Cl phenol
Pyridine
14.54 14.13
14.30 14.03
13.71 14.16
13.96 13.85
125.4 125.9
13.71 13.43
13.47
127.7
4-Cl phenol 4-F phenol Phenol
Pyridine 4-CH3 pyridine 3,5-DiCH3 pyridine
Phenol
4-CH3 pyridine
Phenol
3-CH3 pyridine
125.1
13.28
128.6
4-F phenol
Pyridine
13.43
13.26
128.7
Phenol 3-CH3 phenol
Pyridine Pyridine
13.16
12.71
131.0
4-OCH3 phenol
Pyridine
13.00 12.55
12.55 12.55 12.43
131.4 132.2 132.8
12.14 11.99
134.3 135.1
4-F phenol 4-F phenol Phenol
3-Br pyridine 2-Cl pyridine 3-Br pyridine
11.66 11.46 11.41
Phenol
3-Cl pyridine
11.32
11.88
135.7
Phenol
Pyridazine
11.02
11.53
137.6
10.66 10.39 10.66
10.63 10.78
140.8 141.9
10.28d
10.63 10.26
142.8 145.0
10.22d 10.16d
10.07 10.07
146.2 146.2
Phenol Phenol Phenol Phenol
4-CN pyridine 3-CN pyridine Pyrimidine Pyrazine
4-CH30 phenol Pyridazine 4-CH30 phenol Pyrazine Alcohols and water Methanol
3,5-di-CH3 pyridine
8.74
8.65
155.9f
Methanol Methanol Methanol Water
4-CH3 pyridine 3-CH3 pyridine Pyridine 3,5-di-CH3 pyridine
8.38 8.27 8.20 8.32
157.7 158.6 161.0 164.5
4-C2H5 pyridine
8.14
8.41 8.29 7.98 7.56 7.53
Water
164.7
37
Water
4-CH3 pyridine
7.944
7.45
165.4
Methanol Methanol
3-Br pyridine 3-Cl pyridine
6.67 6.62
7.49 7.42
165.1 165.7
Water
3-CH3 pyridine
7.7ae
7.39
165.9
7.4ae
6.99
169.5
Water a: b: c: d: e: f:
Pyridine
except otherwise stated, the data are from ref.6 same references as for Table 1 ref.36 ref.37 ref.20 PA of OH- : 390.6 kcal mol-' PA of CH30- : 381.4 kcal mol-i ; PA of C2H5O- : 375.3 kcal mol-l. Table 3 Relative frequency shift Av/v and APAfkcal mol-l values for 0-H---N(sp) systems. Solvent : Carbon tetrachloride
Proton donor
Base
102Av/v (exp) (calc.eq.4) APAarb
3-NO2 phenol
2-BuCN
5.84
5.73
143.0
3-Cl phenol
n-BuCN
5.09
5.10
150.0
4-Cl phenol 4-F phenol Phenol
n-BuCN n-BuCN n-BuCN
4.90 4.73 4.46
4.97 4.73 4.43
151.5 155.1 157.4
3-F phenol Phenol
MeCN n-PrCN
4.64
4.41
157.7
4.68
4.41
157.7
3-CH3 phenol
n-BuCN
4.26
4.40
157.8
4-CH3 phenol
n-BuCN
4.41
4.33
158.6
EtCN
4.62
4.31
158.8
Phenol Phenol Phenol
CH2=CHCN MeCN
4.04 4.29
4.05 3.93
161.7 163.0
Phenol
ClCH2CN
3.37
3.13
171.9
Phenol
BrCN
2.82
3.02
173.1
a : data from ref.6 b : same references as in Table 1
38
Table 4 Relative frequency shift Avfv and APA/kcal mol-' values for phenols-anilines systems. Solvent : Carbon tetrachloride Proton donor
Base
102Av/v (exp) (calc.eq.5) APAatb
4-No2 phenol
4-OCH3 aniline
4-No2 phenol 4-No2 phenol 4-No2 phenol 3-No2 phenol 3-Cl phenol 3-Cl phenol 3-Cl phenol 4-Cl phenol 3-Cl phenol
4-CH3 aniline
15.00 14.06
14.70 14.54
118.2 118.9
3-CH3 aniline Aniline
13.64 13.38
14.54 13.93
118.9 121.7
12.15
13.09
125.5
11.77 12.47
12.63 12.25
127.6 129.3
Aniline
10.38
11.48
132.8
Aniline
10.38
11.22
134.0
9.98
10.76
136.1 136.4
Aniline 4-CH3 aniline 4-OCH3 aniline
3-F aniline
Phenol Phenol
4-OCH3 aniline 4-CH3 aniline
10.55 10.14
10.69 10.54
Phenol
3-CH3 aniline
10.00
10.54
137.1
Phenol
9.44
9.92
8.84 9.03
9.59
139.9 141.4
Phenol
Aniline Aniline 4-Cl aniline
9.50
141.8
Phenol
3-F aniline
8.78
9.17
143.3
4-OCH3 phenol
a : data from ref.6 b : same references as in Table 1
137.1
39 Table 5 Relative
frequency
shift Au/v and APA/kcal mol-l values
for 0-Ii---0(sp3) systems. Solvent
Proton donor
Phenol
: Carbon tetrachloride
102Av/v (exp) (calc.eq.6) APAarb
Base
Dimethylsulfoxide
9.70
4-Cl phenol
n-Duty1 ether
4-Cl phenol
Diethyl ether
9.28
140.1
9.00
9.03
141.6
8.31
8.43
145.1
8.43
8.38
145.4
Phenol
Diisopropyl
4-Cl phenol
Tetrahydrofurane
8.73
8.30
145.9
Phenol
n-Butyl ether
7.92
8.03
147.5
4-F phenol
Diethyl ether
7.90
7.82
148.7
4-F phenol
Tetrahydrofurane
8.02
7.69
149.5
Phenol
Diethyl ether
7.51
7.43
151.0
Phenol
Tetrahydrofurane
7.44c
7.29
151.8
ether
4-F phenol
1,4-Dioxane
6.98
6.75
155.0
Phenol
1,4-Dioxane
6.51
6.36
157.3
a : except otherwise stated, the data are from ref.6 b : same references as for Table 1
40
Table 6 Relative frequency shift Au/v and APA/kcal mol-' values for 0-Ii---O(sp*)systems. Solvent : Carbon tetrachloride Proton donor
Base
4-NO2 phenol 4-CN phenol
DMAc
11.98
12.37
DMA
11.96
3-NC2 phenol
11.57 11.36
12.24 11.89
3-CN phenol
DMA DMA
11.67
118.8 120.2
3-Cl phenol
DMA
10.66
10.72
126.1
4-Cl phenol
DMA
10.33
10.53
127.3
3-F phenol
DMA
10.47
10.44
127.9
4-Cl phenol
NMPd DNA
10.34 9.96
128.5
4-F phenol Phenol
10.33 9.89
130.9
NNP
9.56
9.88
131.4
lO*Av/v (exp) (calc.eq.7) APAapb 115.8 116.6
3-OCH3 phenol
DMA
9.80
9.76
132.1
Phenol
DMA
4-CH3 phenol
9.59 9.41
133.2 134.3
I-OCH3 phenol
DMA DMA
9.44 9.06 9.08
9.40
134.4
Phenol
NMP
9.58
9.38
134.5
4-CH3 phenol 3-No2 phenol Phenol 4-Cl phenol 4-F phenol Phenol
NMP
9.30
9.20
Acetone DMFe
7.95 8.14
8.88 8.77
135.6 137.6 138.3
Acetone
6.92
7.17
148.3
6.12
7.03
149.2
6.10f
6.23
154.2
6.05
155.3
Methylethylketone Acetone
4-CH3 phenol
Acetone
4-0CH3 phenol Phenol Phenol
Acetone
6.01f 6.00f
6.04
155.4
Butanal
4.98
5.32
159.9
Propanal
4.71
5.01
161.8
a b c d e f
: : : : : :
except otherwise stated, the data are from ref.6 same references as for Table 1 N,N Dimethylacetamide N-CH3 pyrrolidone N,N-Dimethylformamide ref.41
41
In a relatively small range, the correlations between Au/v
and APA can be
considered as linear. For each family the APA range is indicated in parentheses and the correlation can be written (APA in kcal mol-1) : For 0-H---N(sp3) systems (APA : 105127): lo2 Aviv = 65.6 - 0.35 APA r = 0.972 n = 12 u = 0.8 (2) For 0-H---N(sp2) systems (APA : 124-146): 102 Aviv = 42.8 - 0.23 APA r = 0.953 n = 21 u = 0.5 (3) For 0-H---N(sp) systems (APA : 143-178): lo2 Au/v = 18.6 - 0.09 APA r = 0.971 n = 14 u = 0.2 (4) For 0-H---anilines systems (APA : 118-143): lo2 AVIV = 40.7 - 0.22 APA r = 0.980 n = 17 u = 0.7 (5) For 0-H---0(sp3) systems (APA : 140-157): 102 AVIV = 33.1 - 0.17 APA r = 0.975 n = 12 u = 0.2 (6) For 0-H---0(sp2) systems (APA : 115-162): 102 Au/v = 30.9 - 0.16 APA r = 0.992 n = 25 u = 0.4 (7) Note here that in correlations 2-7, the experimental values of PA rather than the corrected values for the polarisability [42-441 are used. Family dependent correlations between the solubility parameter Ii of Kamlet et al. and the corrected PA have been
42
observed. This dependence has also been observed by ourself in the comparison of the standard Gibbs free energy of hydrogen bonding in solution and the experimental proton transfer parameters [45]. Equations 2-7 clearly show that stronger intercepts (I) are associated with stronger slopes (S), the correlation between these two parameters being linear : I = 2.07 + 179.7 S r = 0.999
(8)
The inverse relationship between the entbalpies of formation of ionic hydrogen bonds and APA has been extensively studied by Meet-Ner and coworkers [10-l 11. In this case also, higher intercepts are roughly associated with steeper slopes. One has to take into account that the enthalpies are not very precise in some cases [43]. The intercepts of equations l-3 are connected to the valence state electronegativity [43-44] which are respectively equal to 1 for N(sp’), 0.60 for N(sp2) and 0.1 for N(sp). One can predict a valence electronegativity of about 0.5 for the aniline derivatives. The same remark also holds for the oxygen bases having valence state electronegativities of 0.20 for O(sp3) and 0 for O(sp2). For the same DPA values the frequency shifts are ordered according to : For N bases :
N(sp3) > N(sp2) 2 anilines > N(sp)
For 0 bases :
WP3) > O(sp2)
This order shows that at the exception of the aniline complexes, the frequency shifts decrease with the enhancement of the s character of the orbital of the non-bonding electrons of the proton acceptor atom. The same trend has been found for the energy of hydrogen bonds f46-491. It is interesting to note here that in the correlation between AHm and APA, there is less distance between the different straight lines [2]. This may be due to the fact that the AvAH values are related to the elongation of the A-H bond brought about by hydrogen bond formation. The stretch of that A-H bond depends mainly on the charge transfer and dispersion terms [50] and not on the other terms such as the electrostatic and repulsion terms contributing to the total hydrogen bond energy. Further, the PA can be considered as the sum of electrostatic, polarization and charge transfer energies [51-521. The contribution of the polarization and charge transfer terms depend on the nature and hybridization of the acceptor atom and amounts to 50 % for N(sp3) atoms and 65% for N(sp) atoms [53]. The greater role of charge redistribution effects (polarization and charge transfer) in determining relative PA and the greater weighting of the electrostatic energies in hydrogen bonds have been emphasized by theoretical calculations [53].
43
The aniline systems deserve special mention. As shown by the results of the present work, the AvOH values are somewhat lower for the anilmes than for the pyridine complexes, notwithstanding the fact that the N atom of free aniline has an hybridization intermediate between sp3 and sp2. The separation into families of the correlation between 8, the scale of hydrogen bond basicities, and AvOH has been explained qualitatively by the influence of the hybridization of the acceptor atom on the energetics of the donor A-H vibration [36,54]. As the A-H bond expands or contracts, this induces back and forth displacements of the free electron pair(s) involved in the hydrogen bond. As the hydrogen of the A-H group vibrates back and forth it pulls and pushes the total electron system of the base with it. This effect depends on the mobility of the electron pair and will be more pronounced for the pyridine complexes where the N atom is involved in the T aromatic system than for the aniline complexes where the N atom occupies an exocyclic position; in this case the direction of the N---HO bond does not coincide with that of the Car-N bond and the effect must be transmitted through the bond.
,--_ ,
a,\*
c-,
<___,+N@'-N-0 C
IS THE RELATION BETWEEN AVOH AND APA LINEAR IN A BROAD DOMAIN ? In a broad range, the correlation between AuOH and APA is not linear but takes an exponential form. This can be shown for O-H---N($)
systems (APA ranging from 100 to
170)
r = 0.983 n = 32 0 = 0.4 (9) FVlrit only one adjustable parameter this expression has a better correlation coefficient than the linear equation 2 (limited to the phenols) with two parameters (r = 0.953). Similar exponential expressions have been discussed for the entbalpies of hydrogen bonds [15-161.
44
It must be noticed here that normal 0-H---N(sp2) hydrogen bond systems characterized by a APA value of zero do not exist. Equation (9) allows to predict a wave number of zero for the stretching vibration of the complex. This value is more realistic than that predicted by equation (3) (about 2100 cm-l). PREDICTION OF THE PA’s OF POLYFUNCTIONAL BASES Equations 2-7 can be applied to estimate the PA’s of polyfunctional bases where several sites are available for hydrogen bond formation. This will be illustrated for nicotinate,
-C
/p \ ,O-CHS
The hydrogen bonds with phenol derivatives are formed on the nitrogen atom and on the oxygen atom of the carbonyl function [55]. The AVOH values for the 4-F phenol complexes are respectively equal to 450 and 220 cm-‘. Application of equation (3) and (7) valuable for 0-H---N(sp2) and 0-H---O(sp2) systems give a PA value of 217.8 kcal mol-l for the N site and of 195.3 kcal mol-l for the 0 site. The experimental value of the PA of unsubstituted pyridine is 220.5 kcal mol-l and the lowering of 2.7 can be accounted for by the inductive and resonance effects of the COOR group, which are both positive [56]. The PA of the C=O site does not greatly differ from that of methyl acetate (196 kcal mol-l) [32]. Nicotinate is protonated on the N-atom and as a consequence, the experimental PA would not greatly differ from 217.8 kcalmol-1. For caffeine,
CH3
complex formation with water or phenols occurs at the two carbonyl functions and this can be proven by the variation of intensities of the two carbonyl stretch vibrations [57J Only
45
one broad VOH band could be observed. As a consequence, the PA’s of the two carbonyl sites cannot be estimated. In the phenols- 1,4,4 trimethylcytosine complexes
“3C\
/AC"3 N
I the hydrogen bond interaction takes place on the carbonyl function [3]. Equation (7) allows to deduce a PA value of 229 kcal mol-l for this site. The experimental PA value of cytosine is 224 kcal mol-l [52-541. This value refers to the protonation on the N atom and as a consequence, these two values cannot be intercompared. Further, the higher value of the computed PA can also be accounted for by the electron donating properties of the three methyl groups of trimethylcytosine. It is noteworthy that the method presented here allows to predict the PA of polyfunctional bases with a relatively good precision. The PA of these molecules can also be extracted from the experimental binding energies of the 1s core electrons of the basic centres [58]. This method, although more general, needs a much more expensive equipment. THE PARADOX OF URACIL The experimental PA of unsubstituted uracil is 209 kcal moll [59-601. Infrared results have shown that in carbon tetrachloride solution, complex formation between 1,3-dimethyluracil and phenols occurs at the O(4) atom [3].
This atom is also the protonation site in the solid state [61] and in Argon matrix [62]. From equation (7) and the results of ref.3, a PA value of 197 kcaJ mol-1 can be estimated for 1,3dimethyluracil. These results are rather unexpected. Indeed, substituting the two hydrogen atoms by two methyl groups should rise the PA by at least 5 kcai mol-l [63-641 and the PA of I,3 dimethyluracil in the gas phase roughly estimated from the substituent effects would be about 215 kcal mol-‘. The great discrepancy between the values of 215 and 197 kcal mol-l suggests that in the gas phase, the uracil derivatives are protonated on one of the N atoms. This conclusion agrees with that of Meot-Ner [59], based on the comparison of the PA of uracil and pyrimidine derivatives. The great difference between the protonated species in the solid state and in the gaseous state is that in the first case, the proton is bounded to the anion [l] and very strong hydrogen bonds are usually formed, whereas in the second case, the protonated species remain essentially free. REFERENCES
1. ADestexhe, M.Sc.Thesis, University of Leuven (1989) 2. ThZeegers-Huyskens, J.Mol.Struct. (Theochem) 135, 93 (1986) 3. OKasende and ThZeegers-Huyskens, J.Phys.Chem. 88,2636 (1984) 4. B.Stymme, H.Stymme and G.Wettermark, J.Am.Chem.Soc. 95, 3490 (1984) 5. M.Jarva,. Thesis, University of Oulu, Finland (1978) 6. M.Joesten and L.J.Schaad, Hydrogen Bonding Marcel Dekker Inc. New York (1974) 7. O.Kasende and Th.Zeegers-Huvskens, SnectroscLett. 17.783 (1984) 8. SFurberg and J.B.Aas;Acta dhem.!&d. A 29, 713 (1975) 9. W.R.Davidson, J.Sunner and P.Kebarle, J.Am.Chem.Soc. 101, 1675 (1979) 10. M.Meot-Ner (Mautner), J.Am.Chem Sot. 106, 1257 and 1265 (1984) 11. M.Meot Ner (Mautner) and L.L.Sieck. J.Phvs.Chem. 89. 5222 (1985) 12. C.V.Speller and M.M& Ner (Maumer) J.Phys.Chem. 89, 5217 (1985) 13. J.W.Larson and T.B.Mc.Mahon, J.Am.Chem.Soc. 106, 517 (1984) 14. J.W.Larson and T.B.Mc.Mahon. J.Am.Chem.Soc. 105. 2944 (1983) 15. ThZeegers-Huyskens, Chem.Phys.lett. 129, 172 (1986) . ’ 16. ThZeegers-Huyskens, J.Mol.Stntct.177, 125 (1988) 17. ThZeegers-Huyskens, Ann.Soc.Sci.Brux. Sec.B 90, 263 (1976) 18. H.P.Honkins Jr.. C.J.Alexander and S.Z.Ali. J.Phvs.Chem. 82. 1268 (1978) 19. MBerthelot, J.F:Gal, C.Laurence and P.C.Maria,J.Chim.Phys.Chim.Biol. 81,327 (1984) 20. O.Kasende and ThZeegers-Huyskens, SpectroscLett. 13,493 (1980) 21. ZDega-Szafran~ A. Hzyneb and M.Szafran, J.Mol.Struct. 240, 159 (1990) 22. B.S.Ault, E.Stembach and G.C. Pimentel, J.Phys.Chem. 79, 615 (1979) 23. ABarnes in Molecular Interactions Vol.1. HRatajczak and W.J.Orville-Thomas Eds. John Wiley & Sons p.287 (1980) 24. A.Allerhand and P.von R.Schleyer, J.Am.Chem.Soc. 85, 371 (1963) 25. W.A.P.Luck, S.Peil, LSchriver and A.Schriver, J. Mol. Struct. 224, 185 (1990) 26. M.GoethaJs Ph.D.Thesis, Leuven (in preparation) 27. W.A.P.Luck and D.Schiiiberg, SpectroscLett. 10, 613 (1977) 28. W.A.P.Luck and D.Schiiiberg, Adv.Mol.Relax.Inter.Proc. 14, 277 (1979) 29. K.Platteborze, J.Parmentier and Th.Zeegers-Huyskens, SpectroscLett. 24, 635 (1991) 30. P.L.Huyskens, H.Marchal and ThZeegers-Huyskens, J.Mol.Struct. 158, 379 (1987) 31. ZLatajka, HRatajczak and ThZeegers-Huyskens, J.Mol.Struct. (Theochem) 164, 202 (1988)
41
32. S.G.Lias, J.F.Liebman and R.D.Levin, J.Phys.Chem.Ref. Data Vo1.13, 695 (1984) 33. M.Fujio, R.T.Mc Iven and R.W.Taft, J.Am.Chem.Soc. 103,4017 (1981) 34. D.Clotman, Ph.D.Thesis, University of Leuven (1969) 35. A.Sucharda-Sobczyk and LSobczyk, Bull.Acad.Pol.Sci. Ser.Sci.Chim. 7,549 (1978) 36. E.M.Amett, E.J.Mitchell and T.S.S.R.Murty, J.Am.Chem.Soc. 96, 3875 (1974) 37. OKasende and Th.Zeegers-Huyskens, J.Phys.Chem 88,2132 (1984) 38. J.E.Bartness and R.T.Mc Iver, Gas Phase Ion Chemistry, Vol.2 M.T.Bowens Ed., Academic Press, New York (1979) 39. T.Iijima and H.KaIincki, Tetrahedon., 35,299 (1979) 40. P.O.Virtanen and M.Jarva, Acta Umversitatis Ouluensis, Ser.A.Chem 3, 3 (1973) 41. C.Thijs and ThZeegers-Huyskens, Spectrochim.Acta 40 A, 307 (1984) 42. R.W.Taft and R.D. Topsom in Progress in Physical Grgamc Chemistry, Vo1.16. R.W.Taft ed., John Wiley & Sons, New York p.16 (1987) and references herein. 43. J.F.Gal and P.C. Maria in Progress in Physical Organic Chemistry, Vol. 17. R.W.Taft ed., John Wiley & Sons, New York p.209 (1990) 44. M.J.Kamlet, R.M.Doherty, J.L.M Abboud, M.H.Abraham and R.W.Taft, Chemtech. 566 (1986) 45. Th.Zeeaers-Huvskens. J.Mol.Lia. 32. 191 (1986) 46. F.B.V& Duij&veldt,‘J.Chem.Phys. 49, 1424 (1968) 47. J.G.C.M.Van Duiineveldt-Van Riidt J.Am.Chem.Soc. 93. a and F.B.Van Duiineveldt. . 5644 (1971) * 48. A.E.Lutskii. A.J.Mitichkin and G.I.Sharemet’eva. Zh.Fiz.Khim. 49. 1374 (1975) \ , 49. J.E.Del Bene, J.Chem.Phys. 58, 3139 (1973) ’ 50. M.M.Szczesniak, Z.Latajka and S.Scheiner, J.Mol.Struc. 135, 179 (1986) 51. H.Umeyama and K.Morokuma, J.Am.Chem.Soc. 99, 1396 (1977) 52. J.Tomasi in Molecular Interachons Vo1.3. HRatajcrak and W.J.Grville-Thomas Eds. John Wiley & Sons p. 119 (1982) 53. P.Kollman and S.Rothenberg, J.Am.Chem.Soc. 93, 1333 (1977) 54. N.J.Kamlet, A. Solomonovici and R.W.Taft, J.Am.Chem.Soc. 101, 3734 (1979) 55. A.Szemik and ThZeegers-Huyskens, J.Mol.Struct. 117, 265 (1984) 56. P.R. Wells, S.Ehrenson and R.W.Taft in Progress in Physical Grganis Chemistry, Vo1.6. A.Streitwieser and R.W.Taft, eds. Interscience Publ. p. 150 (1968) 57. J. De Taeye and Th. Zeegers-Huyskens, J.Pharm.Scienc. 74, 660 (1985) 58. J.Catalan, D.Mo, P.P&ez and M.Ganer, J.Chem.Soc. Perkin Trans.II, 1409 (1982) 59. M.Meot Ner (Maumer) J.Am.Chem.Soc. 101, 2396 (1979) 60. F.Greco, A.Liguori, G.Sindona and N. Uccella, J.Am.Chem.Soc., 112, 9092 (1990) 61. R.Taylor and O.Kennard, J.Mol.Struct. 78, 1 (1982) 62. M.Graindourze, J.Smets, Th. Zeegers-Huyskens and G.Maes, J.Mol.Struct. (in the press) 63. AKatritzky, GBaykut, S. Rachwal, M.Szafran, K.C.Caster and J.Eyler, J.Chem.Soc. Perkin Trans. II 1499 (1989) 64. A.R.Katritzy, M.Szafran and J.Stevens, J.Chem.Soc.Perkin Trans. II 1507 (1989)
Journal of Molecular Liquids 67
(1995)33 -47
CORRELATION BETWEEN THE FREQUENCY SHIFT OF THE STRETCHING VIBRATION OF HYDROGEN BOND COMPLEXES AND THE PROTON AFFINITY The&e Zeegers-Huyskens Department of Chemistry, University of Leuven Celestijnenlaan 2OOF,B-3001 Heverlee Belgium ABSTRACT
The relative frequency shifts of the stretching vibration A-H---B in hydrogen bond complexes are discussed as a function of APA, the difference between the proton affinity of the A- anion and the base. In a relatively small bPA range (f 50 kcal mol-1) the relations can be considered as linear. The slopes and the intercepts of the correlation are ordered according to : N(sp3) > N(sp2) > anilines > 0(sp3) > 0(sp2) > N(sp). In a broad f~ APA domain, the correlation takes an exponential form. The correlations allow to predict the proton affinity of polyfunctional bases where different sites are available for hydrogen bond formation. The correlations suggest that for uracil, the protonation site in the gas phase is not the same as that in the solid state or in low temperature Argon matrix.
INTRODUCTION A student of the laboratory asked me some years ago if it was possible to estimate the proton affinity (PA) of N-CH3-2-pyrimidone which was not determined experimentally.
I answered that the PA can be estimated with a good accuracy from the hydrogen bond parameters [l]. My husband has always been interested to the acid-base behaviour of the hydrogen bonds. I am very happy to dedicate to him this work on the occasion of his retirement. 0167-7322/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-7322 (95) 00864-O
34
The PA of a base B can be extracted from the linear correlation between tbe enthalpies of hydrogen bond formation ( -AHoH& and APA, the difference between the PA’s of the anion A- and that of the base [2]. The error on the experimental determination of AH”HB is usually of the magnitude of & 0.35 kcal mol-l and as a consequence, tbe inaccuracy on the estimation of PA for hydrogen bonds of medium strength, + 3 kcalmoF1 [3]. The PA of a base can also be estimated from the value of the frequency shifts of the “OH stretching vibration. For a very limited number of systems, including phenols complexed with carbonyl bases [3-61, I deduced the following equation AVOH
/cm-l
= 1090 - 1.34 APA /kJ mol-l = 1090 - 5.63 APA lkcal mol-l) (1)
For weak to medium hydrogen bonds (AvOH = 300 cm-l) the experimental error on AvOH = & 5 cm-l. As a consequence, the error on PA (& 0.9 kcal mol-1) is much smaller than that based on the entltalpies. From the experimental frequency shifts of the phenols - N-CH3-2-pyrimidone complexes [3], a value of 212 kcal mol-’ (887 kImoF’) for the PA of the base can be deduced. It must be noticed here that this value refers to the PA of the oxygen atom of the cat-bony1group, the hydrogen bond being formed at this site in carbon tetrachloride solution and the AvOH values refering to the formation of C=O---HO hydrogen bonds. Infrared [7] and X-ray diffraction studies have proven that the N atom is the protonation site in the solid state. This is also likely to be the case in the gas phase, so that the experimental PA of N-CH3-2-pytimidone would probably be different from 212 kcal mol-‘. Owing to the growing interest of the study of the specific interactions in biological molecules, where hydrogen bond acceptor sites are very often present, the relation between the frequency shifts and the PA will be discussed for different hydrogen bond systems. LITERATURE RESULTS AND DISCUSSION Several authors have discussed the relations between the energetics of hydrogen bonds in the gas phase and the PA or APA values [g-16]. Attempts to correlate the hydrogen bond parameters in solution and the PA are much scarcer [17 - 211. Of particular interest is the reduced PA scale defined by Pimentel, widely used in order to predict the proton transfer structure in low temperature matrices [22-231. Owing to the fact that the VOH frequencies depend on the nature of the proton donor, the relative frequency shifts AU/Vwill be considered in this work. Tables 1 - 6 report WV and the APA values for different OH--N and OH--O systems where the N and 0 atoms have different hybridizations. All the data are relative to dilute carbon
35
tetrachloride solution. The frequency of the VO_H___B band is sensitive to the environment and generally increases with the dielectric constant of the medium [24-261. For the water complexes, the decoupled AVOH values have been computed by a previously described method [20] based on the correlation of Luck and .Schi&erg [27-281. It must be pointed out here that in all the complexes considered in this work, the “OH band is relatively broad, sometimes characterized by a fine structure but presents a well-defined maximum. The complexes involving hydroxylic proton donors and the esters are not considered in this work. For the same -AH values, the AVOHvalues are lower than for other carbonyl bases. This can be explained by the fact that the lone pair of electrons of the carbonyl group in cis-position with respect to the alcoxy group is markedly more involved in hydrogen bonding than the tram lone pair. However, as shown by dipole moment studies [30] and by ab initio calculations [31], the angle formed between the C =0 direction and the direction of the hydrogen bond takes values between 40 and 48”, so that the direction of the O-H vibration does not exactly conincide with the direction of the lone pair and smaller AVOH (as compared with ketones and amides) will result. Table 1 Relative frequency shift Au/v and APA/kcal mol-' values for 0-H---N(sp3) systems. Solvent : Carbon tetrachloride Proton
donor
3-HO2 phenol
Base
102Av/, (exp) (calc.eq.2)
APAafb
29.3c
29.0
104.7
28.8d
28.2
106.8
29.ld 28.2d
28.1 27.5
107.3
24.1d
24.3
(C2H5)2NH piperidine
22.7d
24.2
117.9 118.4
23.3d
23.9
119.1
4-Cl phenol Phenol Phenol
(C2H5)2NH piperidine
23.0d 22.7d 22.4d
23.7
119.6
21.9 21.7
125.0 125.5
4-CH3 phenol 4-CH30 phenol
(C2H5)2NH
22.5d
(C2H5)2NH
22.3d
21.4 21.3
126.4 126.7
4-NC2 phenol 4-NC2 phenol 4-CN phenol 3-Cl phenol 3-Cl phenol 4-Cl phenol
(C2H5)3N piperidine (C2H5)2NH (C2H5)2NH piperidine
(C2H5)2NH
108.9
a : The PA of the bases are taken from ref.32 b : The PA of the A- ions are taken from ref.33, taking a PA of 351.4 kcal mol-l for the unsubstituted phenolate ion. c : ref.34 d : ref.35
36 Table 2 Relative frequency shift Aviv and APA/kcal mol-1 values for 0-H---N(sp') systems. Solvent : Carbon tetrachloride Proton donor
Base
102A~/v (exp) (calc.eq.9) APAa#b
Phenols 4-N+
phenol
4-F phenol
Pyridine
19.80
N,N dimethylaminopyrid. 18.00c
17.04
112.7
17.23
115.8 116.5 123.9
3-NC2 phenol
Pyridine
17.20
16.06
3-Cl phenol
Pyridine
14.54 14.13
14.30 14.03
13.71 14.16
13.96 13.85
125.4 125.9
13.71 13.43
13.47
127.7
4-Cl phenol 4-F phenol Phenol
Pyridine 4-CH3 pyridine 3,5-DiCH3 pyridine
Phenol
4-CH3 pyridine
Phenol
3-CH3 pyridine
125.1
13.28
128.6
4-F phenol
Pyridine
13.43
13.26
128.7
Phenol 3-CH3 phenol
Pyridine Pyridine
13.16
12.71
131.0
4-OCH3 phenol
Pyridine
13.00 12.55
12.55 12.55 12.43
131.4 132.2 132.8
12.14 11.99
134.3 135.1
4-F phenol 4-F phenol Phenol
3-Br pyridine 2-Cl pyridine 3-Br pyridine
11.66 11.46 11.41
Phenol
3-Cl pyridine
11.32
11.88
135.7
Phenol
Pyridazine
11.02
11.53
137.6
10.66 10.39 10.66
10.63 10.78
140.8 141.9
10.28d
10.63 10.26
142.8 145.0
10.22d 10.16d
10.07 10.07
146.2 146.2
Phenol Phenol Phenol Phenol
4-CN pyridine 3-CN pyridine Pyrimidine Pyrazine
4-CH30 phenol Pyridazine 4-CH30 phenol Pyrazine Alcohols and water Methanol
3,5-di-CH3 pyridine
8.74
8.65
155.9f
Methanol Methanol Methanol Water
4-CH3 pyridine 3-CH3 pyridine Pyridine 3,5-di-CH3 pyridine
8.38 8.27 8.20 8.32
157.7 158.6 161.0 164.5
4-C2H5 pyridine
8.14
8.41 8.29 7.98 7.56 7.53
Water
164.7
37
Water
4-CH3 pyridine
7.944
7.45
165.4
Methanol Methanol
3-Br pyridine 3-Cl pyridine
6.67 6.62
7.49 7.42
165.1 165.7
Water
3-CH3 pyridine
7.7ae
7.39
165.9
7.4ae
6.99
169.5
Water a: b: c: d: e: f:
Pyridine
except otherwise stated, the data are from ref.6 same references as for Table 1 ref.36 ref.37 ref.20 PA of OH- : 390.6 kcal mol-' PA of CH30- : 381.4 kcal mol-i ; PA of C2H5O- : 375.3 kcal mol-l. Table 3 Relative frequency shift Av/v and APAfkcal mol-l values for 0-H---N(sp) systems. Solvent : Carbon tetrachloride
Proton donor
Base
102Av/v (exp) (calc.eq.4) APAarb
3-NO2 phenol
2-BuCN
5.84
5.73
143.0
3-Cl phenol
n-BuCN
5.09
5.10
150.0
4-Cl phenol 4-F phenol Phenol
n-BuCN n-BuCN n-BuCN
4.90 4.73 4.46
4.97 4.73 4.43
151.5 155.1 157.4
3-F phenol Phenol
MeCN n-PrCN
4.64
4.41
157.7
4.68
4.41
157.7
3-CH3 phenol
n-BuCN
4.26
4.40
157.8
4-CH3 phenol
n-BuCN
4.41
4.33
158.6
EtCN
4.62
4.31
158.8
Phenol Phenol Phenol
CH2=CHCN MeCN
4.04 4.29
4.05 3.93
161.7 163.0
Phenol
ClCH2CN
3.37
3.13
171.9
Phenol
BrCN
2.82
3.02
173.1
a : data from ref.6 b : same references as in Table 1
38
Table 4 Relative frequency shift Avfv and APA/kcal mol-' values for phenols-anilines systems. Solvent : Carbon tetrachloride Proton donor
Base
102Av/v (exp) (calc.eq.5) APAatb
4-No2 phenol
4-OCH3 aniline
4-No2 phenol 4-No2 phenol 4-No2 phenol 3-No2 phenol 3-Cl phenol 3-Cl phenol 3-Cl phenol 4-Cl phenol 3-Cl phenol
4-CH3 aniline
15.00 14.06
14.70 14.54
118.2 118.9
3-CH3 aniline Aniline
13.64 13.38
14.54 13.93
118.9 121.7
12.15
13.09
125.5
11.77 12.47
12.63 12.25
127.6 129.3
Aniline
10.38
11.48
132.8
Aniline
10.38
11.22
134.0
9.98
10.76
136.1 136.4
Aniline 4-CH3 aniline 4-OCH3 aniline
3-F aniline
Phenol Phenol
4-OCH3 aniline 4-CH3 aniline
10.55 10.14
10.69 10.54
Phenol
3-CH3 aniline
10.00
10.54
137.1
Phenol
9.44
9.92
8.84 9.03
9.59
139.9 141.4
Phenol
Aniline Aniline 4-Cl aniline
9.50
141.8
Phenol
3-F aniline
8.78
9.17
143.3
4-OCH3 phenol
a : data from ref.6 b : same references as in Table 1
137.1
39 Table 5 Relative
frequency
shift Au/v and APA/kcal mol-l values
for 0-Ii---0(sp3) systems. Solvent
Proton donor
Phenol
: Carbon tetrachloride
102Av/v (exp) (calc.eq.6) APAarb
Base
Dimethylsulfoxide
9.70
4-Cl phenol
n-Duty1 ether
4-Cl phenol
Diethyl ether
9.28
140.1
9.00
9.03
141.6
8.31
8.43
145.1
8.43
8.38
145.4
Phenol
Diisopropyl
4-Cl phenol
Tetrahydrofurane
8.73
8.30
145.9
Phenol
n-Butyl ether
7.92
8.03
147.5
4-F phenol
Diethyl ether
7.90
7.82
148.7
4-F phenol
Tetrahydrofurane
8.02
7.69
149.5
Phenol
Diethyl ether
7.51
7.43
151.0
Phenol
Tetrahydrofurane
7.44c
7.29
151.8
ether
4-F phenol
1,4-Dioxane
6.98
6.75
155.0
Phenol
1,4-Dioxane
6.51
6.36
157.3
a : except otherwise stated, the data are from ref.6 b : same references as for Table 1
40
Table 6 Relative frequency shift Au/v and APA/kcal mol-' values for 0-Ii---O(sp*)systems. Solvent : Carbon tetrachloride Proton donor
Base
4-NO2 phenol 4-CN phenol
DMAc
11.98
12.37
DMA
11.96
3-NC2 phenol
11.57 11.36
12.24 11.89
3-CN phenol
DMA DMA
11.67
118.8 120.2
3-Cl phenol
DMA
10.66
10.72
126.1
4-Cl phenol
DMA
10.33
10.53
127.3
3-F phenol
DMA
10.47
10.44
127.9
4-Cl phenol
NMPd DNA
10.34 9.96
128.5
4-F phenol Phenol
10.33 9.89
130.9
NNP
9.56
9.88
131.4
lO*Av/v (exp) (calc.eq.7) APAapb 115.8 116.6
3-OCH3 phenol
DMA
9.80
9.76
132.1
Phenol
DMA
4-CH3 phenol
9.59 9.41
133.2 134.3
I-OCH3 phenol
DMA DMA
9.44 9.06 9.08
9.40
134.4
Phenol
NMP
9.58
9.38
134.5
4-CH3 phenol 3-No2 phenol Phenol 4-Cl phenol 4-F phenol Phenol
NMP
9.30
9.20
Acetone DMFe
7.95 8.14
8.88 8.77
135.6 137.6 138.3
Acetone
6.92
7.17
148.3
6.12
7.03
149.2
6.10f
6.23
154.2
6.05
155.3
Methylethylketone Acetone
4-CH3 phenol
Acetone
4-0CH3 phenol Phenol Phenol
Acetone
6.01f 6.00f
6.04
155.4
Butanal
4.98
5.32
159.9
Propanal
4.71
5.01
161.8
a b c d e f
: : : : : :
except otherwise stated, the data are from ref.6 same references as for Table 1 N,N Dimethylacetamide N-CH3 pyrrolidone N,N-Dimethylformamide ref.41
41
In a relatively small range, the correlations between Au/v
and APA can be
considered as linear. For each family the APA range is indicated in parentheses and the correlation can be written (APA in kcal mol-1) : For 0-H---N(sp3) systems (APA : 105127): lo2 Aviv = 65.6 - 0.35 APA r = 0.972 n = 12 u = 0.8 (2) For 0-H---N(sp2) systems (APA : 124-146): 102 Aviv = 42.8 - 0.23 APA r = 0.953 n = 21 u = 0.5 (3) For 0-H---N(sp) systems (APA : 143-178): lo2 Au/v = 18.6 - 0.09 APA r = 0.971 n = 14 u = 0.2 (4) For 0-H---anilines systems (APA : 118-143): lo2 AVIV = 40.7 - 0.22 APA r = 0.980 n = 17 u = 0.7 (5) For 0-H---0(sp3) systems (APA : 140-157): 102 AVIV = 33.1 - 0.17 APA r = 0.975 n = 12 u = 0.2 (6) For 0-H---0(sp2) systems (APA : 115-162): 102 Au/v = 30.9 - 0.16 APA r = 0.992 n = 25 u = 0.4 (7) Note here that in correlations 2-7, the experimental values of PA rather than the corrected values for the polarisability [42-441 are used. Family dependent correlations between the solubility parameter Ii of Kamlet et al. and the corrected PA have been
42
observed. This dependence has also been observed by ourself in the comparison of the standard Gibbs free energy of hydrogen bonding in solution and the experimental proton transfer parameters [45]. Equations 2-7 clearly show that stronger intercepts (I) are associated with stronger slopes (S), the correlation between these two parameters being linear : I = 2.07 + 179.7 S r = 0.999
(8)
The inverse relationship between the entbalpies of formation of ionic hydrogen bonds and APA has been extensively studied by Meet-Ner and coworkers [10-l 11. In this case also, higher intercepts are roughly associated with steeper slopes. One has to take into account that the enthalpies are not very precise in some cases [43]. The intercepts of equations l-3 are connected to the valence state electronegativity [43-44] which are respectively equal to 1 for N(sp’), 0.60 for N(sp2) and 0.1 for N(sp). One can predict a valence electronegativity of about 0.5 for the aniline derivatives. The same remark also holds for the oxygen bases having valence state electronegativities of 0.20 for O(sp3) and 0 for O(sp2). For the same DPA values the frequency shifts are ordered according to : For N bases :
N(sp3) > N(sp2) 2 anilines > N(sp)
For 0 bases :
WP3) > O(sp2)
This order shows that at the exception of the aniline complexes, the frequency shifts decrease with the enhancement of the s character of the orbital of the non-bonding electrons of the proton acceptor atom. The same trend has been found for the energy of hydrogen bonds f46-491. It is interesting to note here that in the correlation between AHm and APA, there is less distance between the different straight lines [2]. This may be due to the fact that the AvAH values are related to the elongation of the A-H bond brought about by hydrogen bond formation. The stretch of that A-H bond depends mainly on the charge transfer and dispersion terms [50] and not on the other terms such as the electrostatic and repulsion terms contributing to the total hydrogen bond energy. Further, the PA can be considered as the sum of electrostatic, polarization and charge transfer energies [51-521. The contribution of the polarization and charge transfer terms depend on the nature and hybridization of the acceptor atom and amounts to 50 % for N(sp3) atoms and 65% for N(sp) atoms [53]. The greater role of charge redistribution effects (polarization and charge transfer) in determining relative PA and the greater weighting of the electrostatic energies in hydrogen bonds have been emphasized by theoretical calculations [53].
43
The aniline systems deserve special mention. As shown by the results of the present work, the AvOH values are somewhat lower for the anilmes than for the pyridine complexes, notwithstanding the fact that the N atom of free aniline has an hybridization intermediate between sp3 and sp2. The separation into families of the correlation between 8, the scale of hydrogen bond basicities, and AvOH has been explained qualitatively by the influence of the hybridization of the acceptor atom on the energetics of the donor A-H vibration [36,54]. As the A-H bond expands or contracts, this induces back and forth displacements of the free electron pair(s) involved in the hydrogen bond. As the hydrogen of the A-H group vibrates back and forth it pulls and pushes the total electron system of the base with it. This effect depends on the mobility of the electron pair and will be more pronounced for the pyridine complexes where the N atom is involved in the T aromatic system than for the aniline complexes where the N atom occupies an exocyclic position; in this case the direction of the N---HO bond does not coincide with that of the Car-N bond and the effect must be transmitted through the bond.
,--_ ,
a,\*
c-,
<___,+N@'-N-0 C
IS THE RELATION BETWEEN AVOH AND APA LINEAR IN A BROAD DOMAIN ? In a broad range, the correlation between AuOH and APA is not linear but takes an exponential form. This can be shown for O-H---N($)
systems (APA ranging from 100 to
170)
r = 0.983 n = 32 0 = 0.4 (9) FVlrit only one adjustable parameter this expression has a better correlation coefficient than the linear equation 2 (limited to the phenols) with two parameters (r = 0.953). Similar exponential expressions have been discussed for the entbalpies of hydrogen bonds [15-161.
44
It must be noticed here that normal 0-H---N(sp2) hydrogen bond systems characterized by a APA value of zero do not exist. Equation (9) allows to predict a wave number of zero for the stretching vibration of the complex. This value is more realistic than that predicted by equation (3) (about 2100 cm-l). PREDICTION OF THE PA’s OF POLYFUNCTIONAL BASES Equations 2-7 can be applied to estimate the PA’s of polyfunctional bases where several sites are available for hydrogen bond formation. This will be illustrated for nicotinate,
-C
/p \ ,O-CHS
The hydrogen bonds with phenol derivatives are formed on the nitrogen atom and on the oxygen atom of the carbonyl function [55]. The AVOH values for the 4-F phenol complexes are respectively equal to 450 and 220 cm-‘. Application of equation (3) and (7) valuable for 0-H---N(sp2) and 0-H---O(sp2) systems give a PA value of 217.8 kcal mol-l for the N site and of 195.3 kcal mol-l for the 0 site. The experimental value of the PA of unsubstituted pyridine is 220.5 kcal mol-l and the lowering of 2.7 can be accounted for by the inductive and resonance effects of the COOR group, which are both positive [56]. The PA of the C=O site does not greatly differ from that of methyl acetate (196 kcal mol-l) [32]. Nicotinate is protonated on the N-atom and as a consequence, the experimental PA would not greatly differ from 217.8 kcalmol-1. For caffeine,
CH3
complex formation with water or phenols occurs at the two carbonyl functions and this can be proven by the variation of intensities of the two carbonyl stretch vibrations [57J Only
45
one broad VOH band could be observed. As a consequence, the PA’s of the two carbonyl sites cannot be estimated. In the phenols- 1,4,4 trimethylcytosine complexes
“3C\
/AC"3 N
I the hydrogen bond interaction takes place on the carbonyl function [3]. Equation (7) allows to deduce a PA value of 229 kcal mol-l for this site. The experimental PA value of cytosine is 224 kcal mol-l [52-541. This value refers to the protonation on the N atom and as a consequence, these two values cannot be intercompared. Further, the higher value of the computed PA can also be accounted for by the electron donating properties of the three methyl groups of trimethylcytosine. It is noteworthy that the method presented here allows to predict the PA of polyfunctional bases with a relatively good precision. The PA of these molecules can also be extracted from the experimental binding energies of the 1s core electrons of the basic centres [58]. This method, although more general, needs a much more expensive equipment. THE PARADOX OF URACIL The experimental PA of unsubstituted uracil is 209 kcal moll [59-601. Infrared results have shown that in carbon tetrachloride solution, complex formation between 1,3-dimethyluracil and phenols occurs at the O(4) atom [3].
This atom is also the protonation site in the solid state [61] and in Argon matrix [62]. From equation (7) and the results of ref.3, a PA value of 197 kcaJ mol-1 can be estimated for 1,3dimethyluracil. These results are rather unexpected. Indeed, substituting the two hydrogen atoms by two methyl groups should rise the PA by at least 5 kcai mol-l [63-641 and the PA of I,3 dimethyluracil in the gas phase roughly estimated from the substituent effects would be about 215 kcal mol-‘. The great discrepancy between the values of 215 and 197 kcal mol-l suggests that in the gas phase, the uracil derivatives are protonated on one of the N atoms. This conclusion agrees with that of Meot-Ner [59], based on the comparison of the PA of uracil and pyrimidine derivatives. The great difference between the protonated species in the solid state and in the gaseous state is that in the first case, the proton is bounded to the anion [l] and very strong hydrogen bonds are usually formed, whereas in the second case, the protonated species remain essentially free. REFERENCES
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