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Proton transfer and secondary deuterium isotope effect in the 13C NMR spectra of ortho-aminomethyl phenols
18 October 1996
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 261 (1996) 283-288
Proton transfer and secondary deuterium isotope effect in the NMR spectra of ortho-aminomethyl phenols
13C
M. Rospenk, A. Koll, L. Sobczyk * Faculty of Chemistry, University of Wroctaw, 50-383 Wroctaw, Poland
Received 17 May 1996; in f'malform 1 August 1996
Abstract The temperature dependence of the deuterium isotope effect on the t3c NMR signals for various positions in the phenyl ring of 2-(N,N-diethylaminomethyl) tetrachlorophenol was studied in the moderately polar solvent CH2C12. The critical temperature, T~ = 245 K, corresponds to the compensation of opposite effects for non-proton-transfer and proton-transfer (PT) forms existing in equilibrium. The UV spectra show that the contribution of the PT form at 245 K amounts to 20 5: 5%. Semi-empirical calculations taking into account the Onsager reaction field allowed us to discuss the role of permittivity and dimerization of polar PT forms in the charge separation.
1. Introduction Measurements of the secondary isotope effect in hydrogen bonded systems are a valuable and promising tool in studies of the charge distribution in molecules [1-4]. This effect is relatively easily measurable and in some cases for carbon atoms in interacting systems it can reach values of 1 ppm. This technique was also successfully applied to studies of ortho-aminomethyi-phenols (Mannich bases) [5]. The intramolecular O - H . . . N hydrogen bonding existing in those molecules causes substantial changes in the charge distribution over the phenyl ring. By choosing appropriate substituents one can reach a
* Corresponding author.
transition ('critical') state where the proton transfer equilibrium appears (Scheme 1). The transition from the non-proton-transfer (HB) to the proton-transfer (PT) state leads to the inversion of the sign of the deuterium isotope effect defined as A = 8t3C(OH) - 8t3C(OD). Because temperature strongly influences the PT equilibrium [6,7] (large positive entropy effect, A H < 0), we decided to investigate the behaviour of the A value for various carbon atoms in 2-(N,N-diethylaminomethyl)tetrachlorophenol (CIaMB) as a function of temperature. We have chosen the tetrachloro derivative because already at not very low temperatures in CH2C12 KpT reaches unity. In the case of the trichloro derivative, where no proton transfer occurs down to 180 K, no temperature effect upon is observed [5]. Values of ~ for both trichloro and
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0009-2614(96)00956-6
M. Rospenk et al. / Chemical Physics Letters 261 (1996) 283-288
284
8
H(D}.
9
8
/ C H 2CH3 j -
CL"
T
CL"~
xCl
9
. (D)H~/CHzCH3 eO'" N
"~
~'Cl
C[
Ct Scheme 1.
tetrachloro derivatives as a function of temperature have already been reported [8]. A huge effect upon the 8 value for carbon atoms in positions 1 and 6 (with opposite sign) for tetrachloro derivatives is observed on cooling.
2. Experimental 2-(N,N-Diethylaminomethyl)-3,4,5,6-tetrachlorophenol was C14MB synthesized and purified according to Ref. [9]. The 13C NMR spectra were recorded on a Bruker AMX 300 spectrometer. The measurements were performed in CD2CI 2 over the temperature range 183-293 K. TMS was used as an internal standard. The values of deuterium isotope effects, A, were determined in two different ways: (i) from independent measurements of undeuterated and deuterated compounds and (ii) from simultaneous measurements of both samples (using two concentric tubes of different diameter). The results obtained on both ways were close. The discrepancies were within the limits of uncertainty in the estimation of the deuteration degree. The value of the deuteration degree estimated by means of IR spectra was in the limits 75-85%. UV spectra were recorded on a Varian Cary-1 spectrophotometer for solutions in CH2C12 over the temperature range 183-293 K, with C = 3 × 10 -4 m o l / d m -3 and d = 0.5 cm.
3. Calculations In order to determine the structure of the studied systems in solution semi-empirical calculations were performed by using the PM3 Hamiltonian [10] of the MOPAC.5 program [11]. The PM3 Hamiltonian was selected because it reproduces better the structure of hydrogen bonded systems than other semi-empirical methods. The solvent effect was accounted according to the MOPAC.5 modification by Szafran et al. [12].
4. Results and discussion The results of measurements of the A value as a function of T for various carbon atoms in CI4MB are presented in Fig. 1. One can see a characteristic (critical) temperature, T¢ = 245 K, at which inversion of the sign of A takes place. At this temperature the contributions of the HB and PT forms compensate each other. It seems understandable that the signs of A for both forms are opposite but their absolute values are not the same. The rate of the proton exchange is high enough to observe only single signals. The contribution of the PT state at 245 K can be evaluated by means of UV spectra. As known [13] the PT form differs in UV spectrum from that of HB form so that it is easy to estimate concentrations
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
285
U!
A
ppm 1.{]
0.5
CB
C4 C6
'C2
HCD.) coordinate
Fig. 3. The double minimumpotential energy curve for the proton and deuteron motion according to Ref. [16]. The deuteration leads to the separation of minima.
-0.5
-1.0
Fig. I. Dependence of the A value for the various carbon atoms in CI4MB on temperature, in CH2C12.
of both forms. Fig. 2 presents the behaviour of UV spectra in CH2C12 at lowered temperatures. Based on these results we can estimate that the contribution of the PT form at 245 K amounts to 205%. The estimations based on the dipole moment measurements and IR studies in CHEf12 at room temperature yield 14 and 26%, respectively [14,15]. This means that the isotope effect for the PT state is markedly larger than that for the HB state. We are not able to determine the A values for both forms because they are temperature dependent (Kr, r is also temperature dependent), but we can estimate, based on maximal values of A that ]ApTViAHBI = 3, so that it agrees E
17@K "~'~ 193K
0.8
0.4 Q2 ' ~ - .
260
~/
x,~..... %¢./ 298K
, 300
,
34~0
r ~ ,nm
Fig. 2. Long wavelength spectrum of CI4MB in CH2CI 2 at various temperatures.
with the contribution of the PT state at 245 K found based on the UV spectra. In Fig. 1 we can see that with the lowering of temperature the absolute values after exceeding the critical point rapidly increase and then at about 200 K again quickly diminish. A drop of the isotope effect at low temperatures is connected presumably with a weakening of the polar O - ... H - N + hydrogen bonds due to a developed charge separation and decrease of the N + - H bond length. We can understand the inversion of the sign of A and dependence of this value on temperature in terms of the theoretical considerations performed by Matsushita and Matsubara [16]. Assuming the double minimum potential for the proton (deuteron) motion they have shown that deuteration of a bridge leads to an increase in the separation of the minima. This means that at the oxygen atom the deuteron is bonded stronger than the proton, while after transfer of the deuteron to the second minimum it is shifted to the nitrogen atom stronger than the proton, as illustrated in Fig. 3. The deuterium isotope effects should be strongest for a symmetrical double minimum potential with sufficiently low barrier. Then the largest Ubbehlode effect [17], the highest anomaly in the isotopic ratio VH/V D [18,19] and the protonic superpolarizability [20] are observed. One should also expect the largest deuterium isotope effect for both HB and PT states when they are in equilibrium (close to an asymmetrical potential). Let us now analyze which factors influence, at low temperature, the potential of hydrogen bonding and the charge distribution. The first factor is generally connected with the electric permittivity of the
286
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
PT
,~H~ kco)Anol -30
^
-50
-70
i
10
i
t"~~-'r~I~3"I~"
30
50
3"3,~
70
,
~.
Fig. 4. Dependenceof the formationenthalpy, A/-/~f,on dielectric permittivity for HB and PT forms assuming different Onsager cavity radii. medium. The permittivity effect upon the A H of the PT reaction can be analyzed by introducing the Onsager reaction field to the calculations [12]. The results of such calculations for C1,MB are presented in Fig. 4, by assuming various cavity radii. The real radius is presumably close to that one resulting from the crystallographic volume (-- 4 ,~3). The results in Fig. 4 show that the permittivity effect is particularly strong for low ¢ values and small Onsager cavities. From Fig. 4 it also follows that at some conditions the PT state can be energetically more favourable than the HB state. This result is in agreement with dielectric studies of the PT equilibrium in hydrogen bonded complexes [21]. However, we should take into consideration limitations of such approach to the macroscopic description of the permittivity effect. As shown elsewhere [22], the environment of the polar molecule cannot be treated as a continuous dielectric. Close to a dipole ordering of the solvent molecules takes place (some sort of freezing), particularly in the case of the highly polar PT form. One could expect that the lowering of temperature causes the ordering to be more and more favourable for the PT form and the charge separation. Deviation from the
Onsager reaction field model should be more pronounced the lower the temperature. In addition, the influence of temperature on the dielectric permittivity, 6', of the solvent contributes to the overall temperature effects. As a rule an increase in ¢' takes place on cooling [23]. Notwithstanding the limitations, calculations of the structure and change in the charge distribution based on the Onsager reaction field yield semiquantitative information on the tendencies which could be expected in the determined A values. In Table 1 the results of calculations of the charge distribution changes going from open (broken hydrogen bonding) to chelate (hydrogen bonded) structure in the gas phase and in CH2CI 2 ( e = 9 . 0 8 ) are collected for both the HB and PT forms of the ClgHB molecule. Although a change in the atomic charge is not the unique factor characterizing the shielding [24], however, when considering the role of hydrogen bonding and the deuterium isotope effect it should be decisive. In fact the A values quite well correlate with calculated charges on atoms if one takes into account that the deuteration weakens hydrogen bonding. Let us notice that for the less polar HB form the medium effect upon the charge distribution can be neglected. The situation is quite different for the polar PT form where the environment possesses a substantial influence on the charge distribution. Let us notice that a change of charge when going from open to hydrogen bonded form for the PT state differs by a sign as compared with the HB state. However, it seems that the difference between the HB and PT states, particularly in the temperature effect on the A value, cannot be interpreted without
Table 1 Differences of atomic charges between open (broken hydrogen bonding) and chelate(hydrogenbonded)form, Aq, for CI4MB Gas phase
Cj C2 C3 C4 C5 C6
CH2CI 2 solution
HB
PT
HB
PT
+ 0.045 -0.019 -0.016 -0.005 +0.003 -0.012
- 0.0244 +0.048 -0.012 +0.005 +0.001 0.000
+ 0.046 -0.016 -0.019 -0.007 +0.005 -0.013
- 0.012 +0.027 -0.009 +0.011 -0.005 0.000
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
287
Table 3 Changes of atomic charges going from PT monomers to PT dimers, Aq, for CI4MB Gas phase
CH2CI 2 solution
+ 0.029 - 0.037 + 0.006 +0.022 - 0.007 - 0.004
+ 0.028 - 0.031 + 0.008 +0.019 - 0.004 - 0.004
C~ C2 C3 C4 C5 C6
e i
allowance for the self-association of polar PT forms. The structural studies [25,26] show that CI4MB crystallizes as PT dimers. The formation of such cyclic pairs should be anticipated just in moderately polar solvents like CH2C12. In fact dielectric studies and measurements of the effective molecular weight [27] indicate the formation of dimers to be of great importance. We have performed calculations of the effect of cyclic dimer formation on the hydrogen bonding and charge redistribution after PT. Going from the crystallographic cyclic dimer the optimization of the structure for CH2CI 2 solution was performed. The crystallographic structure of the PT dimer CI4MB is shown in Fig. 5. The most important parameters characterizing the structure of the cyclic PT dimer are collected in Table 2. From these data it follows that the calculation satisfactorily reproduces the structure of dimers. It is also seen that the transition from the gas phase to a polar solvent does not affect the structure of PT
dimers. Only some increase of the dipole moment should be noticed. The calculated value of the dipole moment in the gas phase for the molecular form of C14MB monomer is 3.88 D while in CH2C12 solution it is 5.14 D by assuming the Onsager cavity radius to be equal to 4.4 A. The experimental values of the dipole moment in cyclohexane and CH2C12 are 4.90 and 5.66 D, respectively [14]. The results of calculations of changes of atomic charges by going from PT monomers to cyclic dimers are given in Table 3. Tremendous effects are seen. Simultaneously, one should notice only a negligible influence of the medium on the charge distribution in the phenolate anion of the PT dimer. The performed calculations are of semi-quantitative character but they allow one to understand the observed trends in the temperature effects upon the A value. Finally, we emphasize that our results of studies on the isotope effect and the influence of temperature closely correspond to the results reported recently by Smimov and coworkers [28,29] who studied the deuterium isotope effect in complexes of
Table 2 Structure of O - ... H - N + hydrogen bridge in cyclic dimers of CI4MB PM3 method
A Hf (kcal/mol)
Intermolecular O - ... H - N + distance ( ~ )
C-Odistance (.~,)
O - ... H distance (/~)
H-N + distance (,~)
/_OHN (deg)
tz (D)
gas phase CH 2CI 2
- 85.953 - 87.302
2.689 2.688
1.260 1.260
1.767 1.768
1.035 1.035
146 146
4.38 5.38
2.637(4) 2.640(4)
1.278(4) 1.279(4)
1.67(4) 1.61(5)
1.00(4) 1.05(5)
163(4) 168(4)
crystal structure a
a Calculations were performed for N,N-diethyl derivative, while crystal structure was determined for N,N-dimethyl derivative.
288
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
carboxylic acids with pyridine labelled by ]SN in liquefied gases at low temperatures enabling the observation of independent HB and PT signals. The isotope effect for the PT form is informative about the displacement of a proton towards the nitrogen atom as the stabilization of the carboxylate anion by the binding of an additional molecule of an acid proceeds. Exactly the same is reached in this work through the continuous lowering of temperature and the formation of PT dimers. References [1] P.E. Hansen, Magn. Reson. Chem. 24 (1986) 903; [2] P.E. Hansen, Prog. Nucl. Magn. Reson. Spectrosc. 20 (1988) 207. [3] P.E. Hansen, J. Moi. Struct. 32 (1994) 79 [4] H.-U. Siehl, Advances in physical organic chemistry, Vol. 23. Isotope effects on NMR spectra of equilibrating systems (Academic Press, London, 1987) p. 63. [5] M. Rospenk, A. Koll and L. Sobczyk, J. Mol. Liquids, 67 (1995) 63 [6] A.I. Kuibida, V.M. Schreiber, Dokl. Akad. Nank SSSR, 250 (1980) 889 [7] A. Koll, M. Rospenk and L. Sobczyk, J. Chem. Soc. Faraday Trans. 177 (1981) 2309 [8] J. Sitkowski, L. Stefaniak, M. Rospenk, L. Sobczyk and G.A. Webb, J. Phys. Org. Chem. 8 (1995) 463 [9] A. Sucharda-Sobczyk and S. Ritter, Pol. J. Chem. 52 (1978) 1955. [10] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 221.
[11] MOPAC.5 QCPE No 455 (1989). [12] M. Szafran, M.H. Karelson, A.R. Katritzky, J. Koput and M.C. Zemer, J. Comput. Chem. 14 (1993) 371. [13] Th. Zeegers-Huyskens and P. Huyskens, in Molecular Interactions, Vol. 2. Proton transfer and ion transfer complexes, eds. H. Ratajczak and W.J. Orville-Thomas (John Wiley, New York, 1980). [14] Z. Pawelka, M. Rospenk and L. Sobczyk, Bull. Soc. Chem. Belg. 96 (1987) 415 [15] M. Rospenk, J. Fritsch and G. Zundel, J. Phys. Chem. 88 (1984) 321 [16] E. Matsushita and T. Matsubara, Prog. Theoret. Phys. 67 (1982) 1. [17] M. Ichikawa, Acta Crystallogr. B 94 (1978) 2074. [18] A. Novak, Struct. Bonding 18 (1974) 177 [19] N.D. Sokolov, M.V. Vener and V.A. Savelev, J. Mol. Struct. 222 (1990) 365 [20] G. Zundel, Trends Phys. Chem. 3 (1992) 124. [21] J. Jad~.yn and J. Malecki, Acta Phys. Polon. A41 (1972) 509. [22] Z. Block and S.M. Walker, Chem. Phys. Lett. 19 (1973) 363. [23] A.A. Maryott and E.R. Smith, Natl. Bur. Stand. Circ. (US) 514 (1951). [24] M. Karplus and J.A. Pople, J. Chem. Phys. 38 (1963) 2803 [25] A. Koll and P. Wolschann, Monatsh. Chem. 1996, 127 (1996) 475. [26] J. Siowikowska, B. Beagley, R.G. Prichard and K. Wo~.niak, J. Mol. Struct., 317 (1994) 99. [27] M. Rospenk and A. Koil, Pol. J. Chem., 67 (1993) 1851. [28] S.N. Smimov, N.S. Golubev, G.S. Denisov, H. Benedict, P. Schah-Mohammedi, H.-H. Limbach, J. Am. Chem. Soc. in press [29] N.S. Golubev, S.N. Smimov, V.A. Gindin, G.S. Denisov, H. Benedict, H.-H. Limbach, J. Am. Chem. SOC. 116 (1994) 12055.
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 261 (1996) 283-288
Proton transfer and secondary deuterium isotope effect in the NMR spectra of ortho-aminomethyl phenols
13C
M. Rospenk, A. Koll, L. Sobczyk * Faculty of Chemistry, University of Wroctaw, 50-383 Wroctaw, Poland
Received 17 May 1996; in f'malform 1 August 1996
Abstract The temperature dependence of the deuterium isotope effect on the t3c NMR signals for various positions in the phenyl ring of 2-(N,N-diethylaminomethyl) tetrachlorophenol was studied in the moderately polar solvent CH2C12. The critical temperature, T~ = 245 K, corresponds to the compensation of opposite effects for non-proton-transfer and proton-transfer (PT) forms existing in equilibrium. The UV spectra show that the contribution of the PT form at 245 K amounts to 20 5: 5%. Semi-empirical calculations taking into account the Onsager reaction field allowed us to discuss the role of permittivity and dimerization of polar PT forms in the charge separation.
1. Introduction Measurements of the secondary isotope effect in hydrogen bonded systems are a valuable and promising tool in studies of the charge distribution in molecules [1-4]. This effect is relatively easily measurable and in some cases for carbon atoms in interacting systems it can reach values of 1 ppm. This technique was also successfully applied to studies of ortho-aminomethyi-phenols (Mannich bases) [5]. The intramolecular O - H . . . N hydrogen bonding existing in those molecules causes substantial changes in the charge distribution over the phenyl ring. By choosing appropriate substituents one can reach a
* Corresponding author.
transition ('critical') state where the proton transfer equilibrium appears (Scheme 1). The transition from the non-proton-transfer (HB) to the proton-transfer (PT) state leads to the inversion of the sign of the deuterium isotope effect defined as A = 8t3C(OH) - 8t3C(OD). Because temperature strongly influences the PT equilibrium [6,7] (large positive entropy effect, A H < 0), we decided to investigate the behaviour of the A value for various carbon atoms in 2-(N,N-diethylaminomethyl)tetrachlorophenol (CIaMB) as a function of temperature. We have chosen the tetrachloro derivative because already at not very low temperatures in CH2C12 KpT reaches unity. In the case of the trichloro derivative, where no proton transfer occurs down to 180 K, no temperature effect upon is observed [5]. Values of ~ for both trichloro and
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0009-2614(96)00956-6
M. Rospenk et al. / Chemical Physics Letters 261 (1996) 283-288
284
8
H(D}.
9
8
/ C H 2CH3 j -
CL"
T
CL"~
xCl
9
. (D)H~/CHzCH3 eO'" N
"~
~'Cl
C[
Ct Scheme 1.
tetrachloro derivatives as a function of temperature have already been reported [8]. A huge effect upon the 8 value for carbon atoms in positions 1 and 6 (with opposite sign) for tetrachloro derivatives is observed on cooling.
2. Experimental 2-(N,N-Diethylaminomethyl)-3,4,5,6-tetrachlorophenol was C14MB synthesized and purified according to Ref. [9]. The 13C NMR spectra were recorded on a Bruker AMX 300 spectrometer. The measurements were performed in CD2CI 2 over the temperature range 183-293 K. TMS was used as an internal standard. The values of deuterium isotope effects, A, were determined in two different ways: (i) from independent measurements of undeuterated and deuterated compounds and (ii) from simultaneous measurements of both samples (using two concentric tubes of different diameter). The results obtained on both ways were close. The discrepancies were within the limits of uncertainty in the estimation of the deuteration degree. The value of the deuteration degree estimated by means of IR spectra was in the limits 75-85%. UV spectra were recorded on a Varian Cary-1 spectrophotometer for solutions in CH2C12 over the temperature range 183-293 K, with C = 3 × 10 -4 m o l / d m -3 and d = 0.5 cm.
3. Calculations In order to determine the structure of the studied systems in solution semi-empirical calculations were performed by using the PM3 Hamiltonian [10] of the MOPAC.5 program [11]. The PM3 Hamiltonian was selected because it reproduces better the structure of hydrogen bonded systems than other semi-empirical methods. The solvent effect was accounted according to the MOPAC.5 modification by Szafran et al. [12].
4. Results and discussion The results of measurements of the A value as a function of T for various carbon atoms in CI4MB are presented in Fig. 1. One can see a characteristic (critical) temperature, T¢ = 245 K, at which inversion of the sign of A takes place. At this temperature the contributions of the HB and PT forms compensate each other. It seems understandable that the signs of A for both forms are opposite but their absolute values are not the same. The rate of the proton exchange is high enough to observe only single signals. The contribution of the PT state at 245 K can be evaluated by means of UV spectra. As known [13] the PT form differs in UV spectrum from that of HB form so that it is easy to estimate concentrations
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
285
U!
A
ppm 1.{]
0.5
CB
C4 C6
'C2
HCD.) coordinate
Fig. 3. The double minimumpotential energy curve for the proton and deuteron motion according to Ref. [16]. The deuteration leads to the separation of minima.
-0.5
-1.0
Fig. I. Dependence of the A value for the various carbon atoms in CI4MB on temperature, in CH2C12.
of both forms. Fig. 2 presents the behaviour of UV spectra in CH2C12 at lowered temperatures. Based on these results we can estimate that the contribution of the PT form at 245 K amounts to 205%. The estimations based on the dipole moment measurements and IR studies in CHEf12 at room temperature yield 14 and 26%, respectively [14,15]. This means that the isotope effect for the PT state is markedly larger than that for the HB state. We are not able to determine the A values for both forms because they are temperature dependent (Kr, r is also temperature dependent), but we can estimate, based on maximal values of A that ]ApTViAHBI = 3, so that it agrees E
17@K "~'~ 193K
0.8
0.4 Q2 ' ~ - .
260
~/
x,~..... %¢./ 298K
, 300
,
34~0
r ~ ,nm
Fig. 2. Long wavelength spectrum of CI4MB in CH2CI 2 at various temperatures.
with the contribution of the PT state at 245 K found based on the UV spectra. In Fig. 1 we can see that with the lowering of temperature the absolute values after exceeding the critical point rapidly increase and then at about 200 K again quickly diminish. A drop of the isotope effect at low temperatures is connected presumably with a weakening of the polar O - ... H - N + hydrogen bonds due to a developed charge separation and decrease of the N + - H bond length. We can understand the inversion of the sign of A and dependence of this value on temperature in terms of the theoretical considerations performed by Matsushita and Matsubara [16]. Assuming the double minimum potential for the proton (deuteron) motion they have shown that deuteration of a bridge leads to an increase in the separation of the minima. This means that at the oxygen atom the deuteron is bonded stronger than the proton, while after transfer of the deuteron to the second minimum it is shifted to the nitrogen atom stronger than the proton, as illustrated in Fig. 3. The deuterium isotope effects should be strongest for a symmetrical double minimum potential with sufficiently low barrier. Then the largest Ubbehlode effect [17], the highest anomaly in the isotopic ratio VH/V D [18,19] and the protonic superpolarizability [20] are observed. One should also expect the largest deuterium isotope effect for both HB and PT states when they are in equilibrium (close to an asymmetrical potential). Let us now analyze which factors influence, at low temperature, the potential of hydrogen bonding and the charge distribution. The first factor is generally connected with the electric permittivity of the
286
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
PT
,~H~ kco)Anol -30
^
-50
-70
i
10
i
t"~~-'r~I~3"I~"
30
50
3"3,~
70
,
~.
Fig. 4. Dependenceof the formationenthalpy, A/-/~f,on dielectric permittivity for HB and PT forms assuming different Onsager cavity radii. medium. The permittivity effect upon the A H of the PT reaction can be analyzed by introducing the Onsager reaction field to the calculations [12]. The results of such calculations for C1,MB are presented in Fig. 4, by assuming various cavity radii. The real radius is presumably close to that one resulting from the crystallographic volume (-- 4 ,~3). The results in Fig. 4 show that the permittivity effect is particularly strong for low ¢ values and small Onsager cavities. From Fig. 4 it also follows that at some conditions the PT state can be energetically more favourable than the HB state. This result is in agreement with dielectric studies of the PT equilibrium in hydrogen bonded complexes [21]. However, we should take into consideration limitations of such approach to the macroscopic description of the permittivity effect. As shown elsewhere [22], the environment of the polar molecule cannot be treated as a continuous dielectric. Close to a dipole ordering of the solvent molecules takes place (some sort of freezing), particularly in the case of the highly polar PT form. One could expect that the lowering of temperature causes the ordering to be more and more favourable for the PT form and the charge separation. Deviation from the
Onsager reaction field model should be more pronounced the lower the temperature. In addition, the influence of temperature on the dielectric permittivity, 6', of the solvent contributes to the overall temperature effects. As a rule an increase in ¢' takes place on cooling [23]. Notwithstanding the limitations, calculations of the structure and change in the charge distribution based on the Onsager reaction field yield semiquantitative information on the tendencies which could be expected in the determined A values. In Table 1 the results of calculations of the charge distribution changes going from open (broken hydrogen bonding) to chelate (hydrogen bonded) structure in the gas phase and in CH2CI 2 ( e = 9 . 0 8 ) are collected for both the HB and PT forms of the ClgHB molecule. Although a change in the atomic charge is not the unique factor characterizing the shielding [24], however, when considering the role of hydrogen bonding and the deuterium isotope effect it should be decisive. In fact the A values quite well correlate with calculated charges on atoms if one takes into account that the deuteration weakens hydrogen bonding. Let us notice that for the less polar HB form the medium effect upon the charge distribution can be neglected. The situation is quite different for the polar PT form where the environment possesses a substantial influence on the charge distribution. Let us notice that a change of charge when going from open to hydrogen bonded form for the PT state differs by a sign as compared with the HB state. However, it seems that the difference between the HB and PT states, particularly in the temperature effect on the A value, cannot be interpreted without
Table 1 Differences of atomic charges between open (broken hydrogen bonding) and chelate(hydrogenbonded)form, Aq, for CI4MB Gas phase
Cj C2 C3 C4 C5 C6
CH2CI 2 solution
HB
PT
HB
PT
+ 0.045 -0.019 -0.016 -0.005 +0.003 -0.012
- 0.0244 +0.048 -0.012 +0.005 +0.001 0.000
+ 0.046 -0.016 -0.019 -0.007 +0.005 -0.013
- 0.012 +0.027 -0.009 +0.011 -0.005 0.000
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
287
Table 3 Changes of atomic charges going from PT monomers to PT dimers, Aq, for CI4MB Gas phase
CH2CI 2 solution
+ 0.029 - 0.037 + 0.006 +0.022 - 0.007 - 0.004
+ 0.028 - 0.031 + 0.008 +0.019 - 0.004 - 0.004
C~ C2 C3 C4 C5 C6
e i
allowance for the self-association of polar PT forms. The structural studies [25,26] show that CI4MB crystallizes as PT dimers. The formation of such cyclic pairs should be anticipated just in moderately polar solvents like CH2C12. In fact dielectric studies and measurements of the effective molecular weight [27] indicate the formation of dimers to be of great importance. We have performed calculations of the effect of cyclic dimer formation on the hydrogen bonding and charge redistribution after PT. Going from the crystallographic cyclic dimer the optimization of the structure for CH2CI 2 solution was performed. The crystallographic structure of the PT dimer CI4MB is shown in Fig. 5. The most important parameters characterizing the structure of the cyclic PT dimer are collected in Table 2. From these data it follows that the calculation satisfactorily reproduces the structure of dimers. It is also seen that the transition from the gas phase to a polar solvent does not affect the structure of PT
dimers. Only some increase of the dipole moment should be noticed. The calculated value of the dipole moment in the gas phase for the molecular form of C14MB monomer is 3.88 D while in CH2C12 solution it is 5.14 D by assuming the Onsager cavity radius to be equal to 4.4 A. The experimental values of the dipole moment in cyclohexane and CH2C12 are 4.90 and 5.66 D, respectively [14]. The results of calculations of changes of atomic charges by going from PT monomers to cyclic dimers are given in Table 3. Tremendous effects are seen. Simultaneously, one should notice only a negligible influence of the medium on the charge distribution in the phenolate anion of the PT dimer. The performed calculations are of semi-quantitative character but they allow one to understand the observed trends in the temperature effects upon the A value. Finally, we emphasize that our results of studies on the isotope effect and the influence of temperature closely correspond to the results reported recently by Smimov and coworkers [28,29] who studied the deuterium isotope effect in complexes of
Table 2 Structure of O - ... H - N + hydrogen bridge in cyclic dimers of CI4MB PM3 method
A Hf (kcal/mol)
Intermolecular O - ... H - N + distance ( ~ )
C-Odistance (.~,)
O - ... H distance (/~)
H-N + distance (,~)
/_OHN (deg)
tz (D)
gas phase CH 2CI 2
- 85.953 - 87.302
2.689 2.688
1.260 1.260
1.767 1.768
1.035 1.035
146 146
4.38 5.38
2.637(4) 2.640(4)
1.278(4) 1.279(4)
1.67(4) 1.61(5)
1.00(4) 1.05(5)
163(4) 168(4)
crystal structure a
a Calculations were performed for N,N-diethyl derivative, while crystal structure was determined for N,N-dimethyl derivative.
288
M. Rospenk et a l . / Chemical Physics Letters 261 (1996) 283-288
carboxylic acids with pyridine labelled by ]SN in liquefied gases at low temperatures enabling the observation of independent HB and PT signals. The isotope effect for the PT form is informative about the displacement of a proton towards the nitrogen atom as the stabilization of the carboxylate anion by the binding of an additional molecule of an acid proceeds. Exactly the same is reached in this work through the continuous lowering of temperature and the formation of PT dimers. References [1] P.E. Hansen, Magn. Reson. Chem. 24 (1986) 903; [2] P.E. Hansen, Prog. Nucl. Magn. Reson. Spectrosc. 20 (1988) 207. [3] P.E. Hansen, J. Moi. Struct. 32 (1994) 79 [4] H.-U. Siehl, Advances in physical organic chemistry, Vol. 23. Isotope effects on NMR spectra of equilibrating systems (Academic Press, London, 1987) p. 63. [5] M. Rospenk, A. Koll and L. Sobczyk, J. Mol. Liquids, 67 (1995) 63 [6] A.I. Kuibida, V.M. Schreiber, Dokl. Akad. Nank SSSR, 250 (1980) 889 [7] A. Koll, M. Rospenk and L. Sobczyk, J. Chem. Soc. Faraday Trans. 177 (1981) 2309 [8] J. Sitkowski, L. Stefaniak, M. Rospenk, L. Sobczyk and G.A. Webb, J. Phys. Org. Chem. 8 (1995) 463 [9] A. Sucharda-Sobczyk and S. Ritter, Pol. J. Chem. 52 (1978) 1955. [10] J.J.P. Stewart, J. Comput. Chem. 10 (1989) 221.
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