J. Phys. Chem. 1984,88, 2132-2137
2132
Infrared Study of Hydrogen-Bonded Complexes Involving Phenol Derivatives and Polyfunctional Heterocyclic Bases. 1. Pyridazine, Pyrimidine, and Pyrazine 0. Kasende and Th. Zeegers-Huyskens* Department of Chemistry, University of Leuven, Celestijnenlaas 200F,B-3030 Heverlee, Belgium (Received: June 1 , 1983; In Final Form: August 30, 1983)
The H-bonded complexes formed between 12 phenol derivatives and pyridazine (l),pyrimidine (2),and pyrazine (3) have been studied by infrared spectrometry (3800-100 cm-I). From the thermodynamic data ( K , -AH, -AS)and the frequency shift of the uOH stretching vibration it can be concluded that the strength of the interaction is 1 > 2 > 3. The complexes of the diazines are compared with those involving pyridine derivatives;the correlation between the -AH values and the pKa or the proton affinity is discussed as a function of solvation effects of the second nitrogen atom of the diazine. The structure of the complexes of 2 (pheno1):l (diazine) stoichiometryis discussed. Several planar.ring modes, mainly the vg, v19, vl modes, are shifted to higher frequencies;these perturbations are lower than those observed in a pyrimidinium ion and are discussed as a function of the amount of electronic charge transferred from the base to the acid. At last, the intermolecular stretching vibration (v,) has been observed in the far-infrared region. For a given base, the wavenumbers of this vibration are not ordered according to the strength of the interaction but depend on the mass of the substituent implanted on the phenolic ring. The corresponding force constant k , is computed by using the Lippincott-Schroeder potential function.
Introduction The interest in the aromatic nitrogen atom as a proton-acceptor atom in hydrogen-bonded complexes is related to the importance of aromatic nitrogen-containing rings in many biological systems. Although the proton-acceptor properties of pyridine have been extensively studied, a review of the data already published reveals that very few thermodynamic and spectroscopic data are available for the diazines and the biological bases derived from pyrimidine.' The stability constant, the enthalpy of complex formation, and the displacement of the vOH stretching vibration have been determined for the complexes of pyridazine, pyrimidine, and pyrazine with unsubstituted phenolz4 and with water.5 The relative base strength of cytosine and uracil toward a proton donor has not been studied in solution and this is probably imputable to the very low solubility of these bases in the usual organic solvents. The first part of this work concerns the thermodynamic and spectroscopic properties of the following four bases complexed with phenol derivatives:
pyridazine
pyrimidine
pyrazine
/
HZN
2-aminop yrimidine
We will successively discuss the thermodynamic parameters (formation constants, enthalpies of complex formation), the structure of the complexes formed by one diazine and two phenol molecules, the effect of complex formation on the internal vibrational modes of the diazines, and the intermolecular stretching mode lying in the far-infrared region. In the second part of this work, the proton-acceptor ability of N-methylated 2- and 4-pyrimidone, and N-methylated uracil and cytosine, whose solubility is sufficient in organic solvents of low polarity, will be investigated. Ab initio S C F calculations6 have shown that the HF-pyridine dimer is more stable than the HF-pyrazine dimer; its greater stability can be attributed to the presence of a more negatively (1) Joesten, M. D.; Schaad, L. J. "Hydrogen Bonding"; Marcel Dekker: New York, 1974. (2) Fritzsche, H. Ber. Bunsenges. Phys. Chem. 1964, 68, 459. (3) Cruege, F.; Girault, G.; Coustal,S.;Lascornbe, J.; Rumpf, P. Bull. SOC. Chirn. Fr. 1970. 3889. (4) Rao, C. N. R.;Jacob, C.; Chandra, A. K. J. Chem. Soc., Faraday Trans I 1982, 78, 3025. ( 5 ) Kasende, 0.;Zeegers-Huyskens, Th. Spectrosc. Lett. 1980, 13, 493. (6) Del Bene, J. E. J. A m . Chem. Soc. 1975, 97, 5330.
charged nitrogen atom in pyridine. The dimer stability of the complexes between water and the three diazines is however not correlated with the total nitrogen elcctron densities.' Further, it must be pointed out that the values of the dipole moments of the complexes of phenol with the three diazines suggest that the preponderant structure is that in which the two aromatic planes are perpendicular; this structure was also confirmed by STO-3G calculations.s
Experimental Section Spectrophotometers. The infrared spectra (4000-400 cm-I) were registered on Perkin-Elmer 325 and 580B (equipped with a Data-Station microprocessor) spectrophotometers. The farinfrared spectra were taken on a Perkin-Elmer 180 spectrophotometer using cells with polyethylene windows. The following solvents were used: carbon tetrachloride (4000-1600 and 1400-1 200 cm-'), tetrachloroethylene (1600-1400 cm-I), carbon disulfide (1 200-600 cm-'), cyclohexane (600-300 cm-I), and benzene (200-100 cm-'). Some infrared-inactive vibrations of pyrazine have been studied by Raman spectroscopy using a Coderg T800 Raman spectrophotometer equipped with a 164 Spectra Physics argon laser emitting a maximum power of 500 mW at 4885 A. The concentrations vary from 0.02 (mid-infrared region) to 0.5 (far-infrared region) M. The temperature of the solution was measured with a thermistor immersed in the cell (accuracy, f l "C). Formation Constants. The formation constants ( K ) of the 1:l complexes have been calculated by using the absorbance of the free phenol derivative at about 3600 cm-l; the phenol concentration varied between and M to prevent self-association and the concentrations of the base ranged from 2 X lo-' to 2 X M. In this limited range, the K values did not depend on the concentration and the reported values result from at least 10 separate determinations. All measurements were performed with CC14 as solvent at temperatures of 298 and 323 K. The standard deviation of K is lower than 5%. The addition of a second phenol molecule to the 1:l complex (Kz)has been computed by a previously described m e t h ~ d ~based .'~ on the variation of an apparent formation constant (Kapp)with (7) Del Bene, J. E. Chem. Phys. 1976, 15, 463. (8) Baraton, M. I.; Besnainou, S.; Gerbier, J. Adu. Mol. Relaxation Interact. Processes 1977, 1 1 , 309. (9) Clotman, D.; Van Lerberghe, D.; Zeegers-Huyskens, Th. Spectrochim. Acta, Part A 1970, 26, 1621. (10) Muller, J. P.; Vercruysse, G.; Zeegers-Huyskens,Th. J. Chem. Phys. Phys.-Chtm. 1972, 1439.
0022-365418412088-2132$01.50/0 0 1984 American Chemical Society
Infrared Study of Hydrogen-Bonded Complexes
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2133
TABLE I: Thermodynamic Data and AUOH Values for the Complexes between Substituted Phenols and Pyridazine' K290K, K323K -AH, -&"298K, phenol dm3 dm ' kJ J K-' Avg~,~ deriv mol-' mol-' mol-' mol-' cm 3,4-di-CH3 29.3 14 24 50 382 4-CH30 37.2 17.6 24 51 3 87 H 46.7 22 25 52 398 4-F 79.9 32.5 26 53 406 4-C1 422 42.5 28 54 102.5 4-Br 118.7 55.1 28 54 4 26 121.7 48.9 28 55 430 3-F 145.1 56.9 29 55 434 3-C1 3-Br 146.8 59.4 29 56 444 3,4-di-C1 313.4 117.1 30 51 462 3-N02 370.5 157.8 32 58 470 3,5-di-C1 341.5 110.2 32 59 477 ' s = CCl, Broad band; error of the absorption maxima, +5 cm-'
.
TABLE 11: Thermodynamic Data and AUOH Values for the Complexes between Substituted Phenols and F'yrimidine' ~2ssK p 3 K , -AH, - ~ s 2 9 8 K , phenol dm3 dm3 kJ JK-' deriv mol-' mol-' mol-' mol-' cm 3,4-di-CH3 12.1 6 21 50 372 4-CH3 12.8 7.1 22 51 373 3 85 H 17 8.6 23 52 4-F 22.7c 9.7 23 53 393 4-C1 28.8 13.3 25 55 420 4-Br 31 13.5 25 55 417 26 56 419 3-F 37.8 16.2 3-C1 43 26 56 4 25 18.7 3-Br 48 19 26 57 436 3,4-di-C1 71 29 28 60 445 3-NO, 81 34 29 61 453 3,5-di-C1 82 33 30 61 46 2 ' S = CCl,. Broad band; error of the absorption maxima, +5 cm-'. Gurka and Taft', quote a value of 22.5 dm3 mol-' at a temperature of 295 K. the phenol concentration. If C, represents the concentration of the free phenol derivative determined from the absorbance of the lvoH stretching band, it can be shown that 1
Kapp = -I - K2 2Ca + KappC2 K 2Ca + KappC2 When the first term of this equation is plotted against Kapp/(2Ca KapPC:), one obtains a straight line whose slope is equal to 1/K and whose intercept is equal to -K2. The K2values were computed by using 10 different phenol concentrations ranging from 2 X to 0.5 X M. The standard deviation of K2 is about 10%.
+
Results and Discussion Thermodynamic Data and AvOH Values. Tables I-IV list the formation constants ( K ) determined at 298 and 323 K, the enthalpies and entropies of complex formation, along with the frequency shift of the luoH vibration for the 1:l complexes between phenol derivatives and pyridazine, pyrimidine, 2-aminopyrimidine, and pyrazine. Owing to the weak solubility of 2-aminopyrimidine, the AuOHvalues could not be measured with precision; the AvOH value for the complex of unsubstituted phenol, obtained with accumulation of the spectra, is about 400 cm-'; for this complex, the weak displacement of the vNH2 bands (5-10 cm-') suggests that complex formation takes place on the nitrogen atom of the ring, I 1 when the hydrogen bonds are formed on the exocyclic NH2 group, a frequency shift of more than 50 cm-' is observed.12 From
TABLE 111: Thermodynamic Data for the Complexes between Substituted Phenols and 2-Aminopyrimidine' ~ 2 9 8 K ~323K -AH, - ~ ~ 1 9 8 K , phenol dm3 dm kJ JK-' deriv mol-' mol-' mol-' mol-' 3,4-di-CH3 30 14.6 23 49 4-CH30 34 16.8 24 50 H 42 21 25 51 4-F 54 24 25 52 4-C1 78 34 27 54 4-Br 84 35 21 54 3-F 92 39 28 55 3-C1 106 39.5 28 56 3-Br 110 43 28 56 3,4-di C1 165 68 29 58 3-NO, 218 82 31 59 3,5-di-C1 220 89 31 60 a
s = cc1,.
TABLE IV: Thermodynamic Data and AUOH Values for the Complexes between Substituted Phenols and Pyrazine' ~298K, K323K, -~S298K! phenol dm3 dm3 kJ JK-' A ~ g q , ~ mol-' mol-' mol-' cmderiv mol-' 3,4-di-CH3 4-CH30 H 4-F 4-C1 4-Br 3-F 3-C1 3-Br 3,4-di-C1 3-NO, 3,5-di-C1 ' s = cc1,. cm-'
.
8.5 9.3
4.4 21 52 365 4.7 21 53 367 11 5.9 22 54 371 6.2 13 22 54 3 80 16.3 8.5 24 56 392 8.6 17.9 24 56 400 20.8 9.2 24 56 408 22.3 10 25 57 419 22.7 10.5 25 57 423 34.3 15 26 59 433 45.4 18.8 27 60 445 48.5 21.5 27 60 450 Broad band; error of the absorption maxima, t 5
the experimental results, it can be concluded that the strength of the interaction follows the order pyrazine
< pyrimidine < 2-aminopyrimidine < pyridazine
With the exception of 2-aminopyrimidine, whose pKa = 3.67, this order follows the basic strength determined in water (see further discussion). The K values are proportional to the acidity of the phenol derivatives and the following equations were obtained by the mean least-squares method with a correlation coefficient between 0.99 and 0.98: pyridazine log K298K= 6.77 - 0.50(pKa) log K323K= 5.88 - 0.45(pKa) log K298K = 5.29 - O.40(pKa) log K323K= 4.47 - 0.35(pKa)
(2)
2-aminopyrimidine log K298K = 5.73 - 0.41(pKa) log K323K= 4.78 - 0.36(pKa)
(3)
pyrazine log K298K= 4.55 - 0.35(pKa) log K323K= 3.84 - 0.31(pKa)
(1 1) JacquB-Lafaix, A.; Josien, M. . J. Chim. Phys. Biol. 1965, 684 and results of the present work. (12) Lichtfus, G.; Zeegers-Huyskens,Th. Spectrochim. Acta, Part A 1972, 28, 2081.
(1)
pyrimidine
(13) Gurka, D.;
Taft, R. W. J. A m . Chem. SOC.1969, 91, 4974.
(4)
2134 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
0
Kasende and Zeegers-Huyskens
0
30 c
25
/
20
so!-
A l
l
1
l
!
l
l
l
l
~
l
I
470
360
Figure 1. -AH (kJ mol-') as a function of AuoH (cm-I) for the complexes between phenol derivatives and (0)pyridazine, (0)pyrimidine, and (A)
pyrazine. TABLE V: pK, and Proton Affinity (PA) of the Diazines and Some Pyridine Derivatives and Complexation Enthalpy (-AH) for the Complex with Unsubstituted Phenol PA,g
nitrogen base pyridazine pyrimidine pyrazine pyridine 3-methylpyridine 4-methylpyridine 3,5-dimethylpyridine 3-bromopyridine
kcal mol-' 213.8a 208.6' 206.4' 218.1b 220.5' 221.3c 223.6d
PK, 2.30 1.23 0.67 5.17 5.68 5.98 6.15 2.84 3-cyano pyridine 206.8& 1.36 4-cyanopyridine 207.6& 1.90 a Reference 16. Reference 17. ' Reference 18. 19. e This work. Mean values quoted from ref 1. affinities are expressed in the usual units (kcal mol-'); ience, the enthalpies are expressed in the same units.
\
, I
3CN
*fCN 2
l
3
l
1
I
l
1
5
l
L
6
Figure 2. -AH (kcal mol-') for the complex of unsubstituted phenol as a function of the pK, of the base. Line A: diazines (0)pyridazine, (0) pyrimidine, and (A) pyrazine. Line B: ( 0 )pyridine derivatives.
7.0 I
-AH, kcal mol-'
6.0Y 5.43= 5.26e 6.5f 6.6f 6.7f 6.gf 5.4f 4.8f 4.9f Reference The proton for conven-
showing that the intercept and the slope decrease with increasing temperature; a similar effect was noticed for hydrogen-bonded complexes involving oxygenI4 and sulfur atorns.l5 The slopes of the correlations relating the influence of a substituent implanted on the phenol molecule to the equilibrium constant also increases with the strength of the interaction or in other words with decreasing 0. .N distance. Further, as shown by Figure 1, where -AH has been plotted against AuoH, the points relative to the three diazines are approximately situated on the same straight line:
5 5
--
PA(kcal/rnoi'),
-
-AH(kJ mol-') = 0.092AuoH(cm-') - 12.59 ( r = 0.950)
(5)
suggesting that the complexes of the three diazines are closely related. Starting from this point of view, it is interesting to compare the complexes studied in this work with those formed between the same proton donors and pyridine derivatives. Table V lists the enthalpies of complex formation of some heterocyclic nitrogen bases, taking unsubstituted phenol as reference acid along with the pK, of the bases. As can be seen from Figure 2, where -AH has been plotted vs. pK,, the complexes of the diazines and of the pyridine derivatives are situated on two different straight lines. (14) Stymme, B. Ph.D. Thesis, Royal Institute of Technology of Stockholm, 1978. (15) Reyntjens-Van Damme, D.; Zeegers-Huyskens,Th. J. Phys. Chem. 1980, 84, 282. (16) Del Bene, J. E. J . Am. Chem. SOC.1977, 99, 3617. (17) Amett, E. M.; Chawla, B.; Bell, L.; Taagepera, M.; Taft, R. W. J. A m . Chem. SOC.1977, 99, 5729.
Figure 3. -AH (kcal mol-') as a function of the proton affinity of the base (kcal mol-I): (0)pyridazine, (0)pyrimidine, (A)pyrazine, and ( 0 ) pyridine derivatives.
For the same pK, value, the complexes of the diazines are more stable (by about 0.8 kcal mol-') than the complexes of the pyridines. Figure 3, where -AH has been plotted against the gasphase proton affinity (PA), shows that a single correlation is found for the diazines and the pyridine derivatives. As shown by one of us,2othe gas-phase thermodynamic parameters correlate much better with the hydrogen-bonding data than the aqueous values. The hydrogen-bonding Gibbs free energy changes have been correlated to the proton affinities for complexes involving pyridine derivatives'* but marked deviations are observed for sterically hindered base; these deviations are strongly attenuated when using, instead of the -AGO values, the -AH(or AuOH)values.20 The two intercepts observed in Figure 2 can be explained, at least partly, by the formation in aqueous solution of hydrogen bonds between the two nitrogen atoms of the ring and the surrounding water molecules:
Hehre, W. J.;
(18) Hopkins, H. P.; Alexander, C. J.; Zakir, Ali S . J . Phys. Chem. 1978,
82, 1228. (19) Aue, D. H.; Webb, H. M.;
Bowers, M. T.; Liotta, C . L.; Alexander, C. J.; Hopkins, H. P. J . A m . Chem. SOC.1976, 98, 854.
(20)
Zeegers-Huyskens, Th. Ann. SOC.Sci. Bruxelles
1976, 90, 263.
Infrared Study of Hydrogen-Bonded Complexes
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2135
TABLE VI: K and Kza Values for the Complexes between the Diazines and Four Phenol Derivativesa
3,4-dichlorophenol base p yridazine
pyrimidine
34.3
pyrazine a
K 26 9 71
K , is defined as CA,B/CABCA,C ,
derivative, respectively.
3-nitrophen01
K* 21.8 12.2
K 370 81
6.7
3,5-dichlorophenol K
K2 22.8 14.3
45.4
398 82 48.5
8.8
K, 23.1 15.1 9.6
3,4,5-trichlorophenol K 713 93
K, 24 .I 18.9
65
14.3
B, CAB,and CA being the concentrations of the 2: 1 complex, the 1:1 complex, and the free phenol
T = 298 K; S = CC1,. All values are in dm3 mol-'.
The formation of a hydrogen bond on the N(2) atom diminishes the electronic density on the N( 1) atom which could be less able to undergo protonation in a second step. This effect does not exist in the gas phase, where only one proton affinity has been experimentally determined; the proton affinity of the second basic site must be obviously lower. As a matter of fact, the data obtained from STO-3G calculations show that, in the phenol-pyridazine complex, the second nitrogen atom experiences a decrease in negative charge; 21 as a consequence, this atom would be less able to form a hydrogen bond with another phenol molecule. From this point of view, it seemed interesting to us to investigate the stability along with the two possible structures of the 2:1 complexes between the three diazines and some phenol derivatives. Table VI lists the addition constants of a phenol molecule on the 1:l complex; the K values of Tables I-IV are added for comparison. As shown by the values reported in this table, the K2 values are, for all the studied systems, lower than the K values. The following equations have been calculated by the mean least-squares method with a correlation coefficient higher than 0.99: pyridazine log KzZgsK= 1.85 - 0.059(pKa)
(6)
log K2298K= 3.08 - 0.23(pKa)
(7)
pyrimidine pyrazine log K2298K= 4.21 - 0.39(pKa)
(8) It is worthwhile to note that the slopes of these equations do not increase with the strength of the interaction as was the case for the log K-pKa correlations; on the contrary, the slopes and intercepts become smaller than when the basic strength of the diazine increases. The two possible structures for the 2:1 complexes are structure I, where the two phenol derivatives are hydrogen bonded,
I
I1
and structure 11, where the two nitrogen atoms of the diazine are involved in a hydrogen bond with a phenol molecule. From the theory of Huyskens,22it can be concluded that a first hydrogen bond involving a given site of a molecule or ion weakens the reactivity of the neighboring sites of the same nature whereas it enhances the electron-donor or -acceptor power of the adjacent sites of opposite nature. As a consequence, the K2 values for complexes of type I must be higher than the dimerization constants of the phenol derivatives; as shown in earlier ~ o r k s , these ~,~~ constants do not markedly depend on the substituent implanted on the phenolic ring and decrease weakly with increasing acidity. Consequently,20the slope of the relationship log K2-pKa must be sensibly lower than that of the correlation log K-pKa for complexes of type I; for the complexes between phenols and tetramethylurea characterized by this type of structure, the slope of log K-CCTH (21) Baraton, M. I. Th6se de Doctorat d'Etat, UniversitB de Limoges, Limoges, France, 1979. (22) Huyskens, P. L. J . Am. Chem. Soc. 1977,99, 2578.
Figure 4. Infrared spectrum in the uOH range of pyrazine-phenol solutions ( S = CCIJ: Cpyramc = 0.01 M; Cphenal = 0.01 (spectrum a) and 0.04 (spectrum b) M.
( x u Hbeing the sum of the Hammett substituent constants) is approximately 10 times greater than that of the relationship log K2-CaH.10 Comparison of the slopes of eq 1 and 5 of this work (respectively, -0.50 and -0.059) strongly suggests that the pyridazine systems probably possess the structure I. The behavior of the pyrimidine and pyrazine seems to be different; the ratio of the slopes of the correlation log K-pKa and log K2-pKa is much lower, about 1.7, for the pyrimidine systems and 1 for the pyrazine complexes. Approximately the same ratio (1.5) was obtained for the tetraalkylammonium-phenol hydrogen-bonded complexes23which are characterized by a structure of type 11. The results of this work strongly suggest that the 2:l adducts of pyrimidine and pyrazine possess the same structure. The structural differences between pyridazine and the two other diazines are probably ascribable to a steric inhibition between the two phenol molecules when they are complexed on two adjacent nitrogen atoms; this steric hindrance is strongly attenuated in a linear Ne .H-0 chain. The infrared spectrum of the pyrazine-phenol complexes in the vOH range (3700-3100 cm-') does not show any variations with the phenol concentration. As shown in Figure 4, relative to pyrazine-phenol solutions,a broad band characteristic of the v0H-N vibration is observed at 3240 cm-' (Av = 370 cm-') for equimolecular solutions (spectrum a); in excess of phenol, a second band of weaker intensity appears at 3380 cm-' (Av = 230 cm-I) (spectrum b), probably ascribable to the formation of a second OH. .N bond. The smaller frequency shift is due to the decreased electronic density on the second nitrogen atom. This band could not be observed for the pyrimidine systems owing to the overlapping of the vOH...N absorption with overtones of the base. Perturbation of the Internal Vibrational Modes of the Diazines. The phenol bands lying at 1342 and 1177 cm-', involving a aOH in-plane deformation mode, are shifted to 1388 and 1235 crn-';
-
-
(23) Rulinda, J. B.; Zeegers-Huyskens,Th. Adv. Mol. Relaxation Interact. Processes 1979,14, 203.
2136 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
Kasende and Zeegers-Huyskens
TABLE VII: Fundamental Vibrations (cm-') of Pyridazine, Pyrimidine,and Pyrazine Perturbed by Complex Formation with Unsubstituted Phenol
assignment and symmetry
pyr idazine
free
complexed
1570 1564 1443 1411
2840,2732, 2604, 2504 1571 1583 1449 1415
VOH-.N
vas, Ai? Agb (Big)
v8b, B i
B~ ( h u ) A, ( B i d Ai (Ag)
"igb> V1gaj
Vga,
V153B1
cB3U)
VlWA,
(%u)
A, (Ag) (Big) VI1' Bz (BZLl) V 6 a ' B l (B1g) v6b, Ai (A,) V I ,
vioa, Bz
'16b, B2
(Bzu)
free
pyrimidine complexed
C
C
1060
1063
C
C
966 746
973 750
C
C
664
667
C
C
C
C
364
366
621 34 3
628 395
2840,2737, 2600, 2504 1580 1530
1574e 1523e C
C
1411 1227
1415 1223
d
d
1018 1012e
1027 1022d
C
C
C
C
718
716
785
781
a C, symmetry group (pyridazineand pyrimidine). Dlh symmetry group (pyrazine). a phenol band. e Raman spectra (S = CCl,, concentration of phenol and pyrazine = 1 M).
TABLE VIII: Av Values for Some Pyrimidine Vibrations in the
free
2840,2732, 2600,2504 1579 1568 1476 1405 1137 1165 d 928
1569 1566 1460 1400 1136 1158 1069 989
pyrazine complexed
C
C
598e 417
618 428
Too weak to be observed.
Overlapping with
100 I
Complex with Phenol and in Pyrimidinium Chloride Av, cm-'
assignment 8a 8b 19b 19a 11 a
pyrimidinium chloridea
pyrimidinephenol
+55 +39
+10 +2 +6 +5 -2
+I1 +57 -23
130
From ref 33 using the wavenumbers of the free molecules of
this work.
the absorption observed a t 1258 cm-' involving mainly a vc4 stretching mode is shifted by about 10 cm-' by complex formation with pyrimidine. These perturbations are similar to those observed for other H-bonded complexes24and will not be discussed here. The infrared and Raman spectra of the diazines have been studied in earlier works; 25-27 the assignment of the vibrational modes in the free and complexed molecules is reported in Table VII. To the low-frequency side of the main uOH ...N absorption, a set of bands of lower intensity are observed at about 2840,2730,2600, and 2500 cm-'; these wavenumbers are practically independent of the nature of the base. This is in agreement with the theory of Hall and Wood,2*who have shown that the substructure of the v,(OH) band in hydrogen-bonded systems involving phenol derivatives is largely due to Fermi resonance with overtone and combination bands of the proton-donor moiety. The results of Table VI1 show that several planar ring modes (vg, u19, vlz, vlr v6) are shifted to higher frequencies, the perturbation of the in-plane (Q,,v15) and out-of-plane (vI1, vlOa) H-bonding modes being generally smaller. Some of these vibrations have been observed for the complexes between pyrimidine and pyridazine with water or methanol.29 The shifts of the skeletal vibrations to higher frequencies can be explained by an increase of the overlap populations in the ring, clearly evidenced for the pyridine-water complex and for the pyridinium ion; 3o for this ion (24) Dorval, C.;Zeegers-Huyskens, Th. Spectrochim. Acta, Part A 1973, 29, 1805. (25) Lord, R. C.; Marston, A. L.; Miller, F. A. Spectrochim. Acta 1957, 9, 113. (26) Foglizzo, R.; Novak, A. J . Chim. Phys. Phys.-Chim. Biol. 1967, 64,
1484. (27) Lafaix, A.; Lebas, J. M. Spectrochim. Acta, Part A 1970, 26, 1243. (28) Hall, A,; Wood, J. L. Spectrochim. Acta, Part A 1967, 23, 1257. (29) Takahashi, H.; Mamola, K.; Plyler, E. K. J . Mol. Spectrosc. 1966, 21, 217. (30) Adam, W.; Grimison, A.; Hoffman, R.; de Ortiz, C. Z. J. Am. Chem. SOC.1968, 90, 1509.
120
-60 1
'
c m-'
I
I
175 150 125 100 Figure 5. Far-infrared spectrum of the complex 3-nitrophenol (C = 0.05 M) and (a) pyridazine (C = 0.5 M) or (b) pyrazine (C = 0.5 M). S =
C6Hs. The spectra have been obtained by subtracting from the spectra of the ternary solutions the spectra of the individual components and of
the solvent. an increase of the Cl-C2 bond order has been calculated and an increase of the frequencies of the vga and vlgb modes has been p r e d i ~ t e d . ~ Very ' few theoretical results are available for the H-bonded and protonated diazines. It has however been shown that the general effect of protonation of a diazine is to increase the total electron density at the adjacent carbon atoms.32 From this point of view, it is interesting to compare the wavenumbers of some sensitive vibrations in the pyrimidinium ion33and in the pyrimidine-phenol complex. The frequency shifts are listed in Table VIII. These data indicate that the shifts of the bands of the proton-transfer complex are approximately 10-1 5 times greater than those of a normal hydrogen bond. These results must however be taken with caution; as shown by N ~ v a the k ~ ring ~ vibrations 8b, 19b, and 19a are shifted to higher frequencies-at least partly-by a mechanical coupling with the 8"t mode; the comparison between the Au values of the pure 8a mode is much more valuable. These spectroscopic peturbations can be related to the (31) Sabin, J. R. J . Mol. Struct. 1972, l J , 33. (32) Adam, W.; Grimison, A,; Rodriquez, G. Tetrahedron 1967,23,2513. (33) Foglizzo, R.; Novak, A. Spectrochim. Acta, Part A 1970, 26, 2281.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2137
Infrared Study of Hydrogen-Bonded Complexes
TABLE X: k o and ~ k , Values (N m-l) Computed by the
TABLE E: Far-Infrared Data for the Complexes between Phenol Derivatives and the Diazinesa
Lippincott and Schroeder Potential Function pyridazine pyrimidine phenol deriv kOH k, kOH ko 3.4-di-CH614 17.72 620 16.58 4:CH30 16.42 613 18.06 6 20 608 19.10 phenol 17.72 6 14 4-F 606 19.65 609 18.93 4421 601 21.17 601 20.97 4-Br 597 21.97 602 20.59 3-F 595 22.38 602 20.78 3421 594 22.80 598 21.77 591 3-Br 23.65 595 22.59 3,4-di-C1 5 84 25.45 5 89 24.09 3-N02 581 26.40 5 80 26.64 3,S-di-Cl 577 581 27.37 26.40 3,4,5-tri-C1 568 30.24 572 28.90
u,, cm"
phenol deriv pyridazine pyrimidine pyrazine 3,4-di-CH3 117 116 114 4-CH30 118 116 115 phenol 132 123 122 4-F 128 122 120 4-C1 124 119 118 4-Br 121 120 117 3-F 130 122 120 3-C1 125 121 119 3-Br 130 119 118 3,4-di-C1 127 118 113 3-N02 130 122 120 3,5-di-C1 117 113 108 3,4,5-tri-C1 118 113 110 Registered in extended scale. Error of the absorption maxima, 22 cm-'.
105
I
/
30
110
115
120
125
130
135
Figure 6. -AH (kJ mol-') as a function of v, (cm-I): (A) phenol, (B) 3-bromophenol,( C ) 3,5-dichlorophenol;(0)pyridazine, ( 0 )pyrimidine, (v)pyrazine, ( 0 )pyridine (v, taken from ref 34).
amount of electronic charge transferred from the base to the proton: 0.519 e; l6 this value is much higher than the charge transfer in the pyrimidine-phenol complex (about 0.035 e).21 For the 2-aminopyrimidine-3,4,5-trichlorophenolcomplex, the v,(NH2) and vs(NH2) bands are shifted by 7 and 11 cm-' to lower , 6 is shifted by 10 cm-' to higher frequencies, frequencies and the clearly indicating that complex formation takes place on the nitrogen atom of the ring. The perturbations of the in-plane and out-of-plane skeletal modes are very similar to those observed for the pyrimidine complexes. Intermolecular Stretching Vibration vu. The wavenumbers of the intermolecular stretching vibration vu are indicated in Table IX, and two of the spectra are reproduced in Figure 5. These experimental data indicate that the vu values relative to the complexes between a given base and the series of proton donors do not correlate with the strength of the interaction; the highest vu values are obtained for the complexes of unsubstituted phenol whose enthalpies of complex formation are obviously smaller than those obtained for 3,5-dichlorophenol systems. For these last complexes, the smaller vu values were observed. These experimental features have been explained in earlier ~ o r k s by ~ ~the9 ~ ~ (34) Lichtfus, G.; Zeegers-Huyskens, Th. J. Mol. Sfruct. 1971, 9, 343.
pyrazine kOH
k,
626 625 617 615 607 608 605 602 601 594 589 5 86 577
15.06 15.21 17.06 17.55 19.47 19.10 19.83 20.59 20.97 22.80 24.09 24.99 27.62
increasing mass of the substituents implanted on the phenolic ring. However, for a given phenol complex, the vu values are ordered according to the strength of the interaction. This is illustrated in Figure 6 for three different phenol derivatives;it appears clearly that the same vu value can be obtained for quite different values of the enthalpy of complex formation. It is also worth noting that the point relative to the complex of unsubstituted phenol with pyridine (characterized by about the same mass as the diazines) is situated on curve A. The force constants of the O H (koH)and intermolecular (k,) bonds have been calculated by the undimensional potential function of Lippincott and S~hroeder'~ using a OH bond dissociation energy of 107 kcal mol-', a repulsion constant of 4.8 X lo8 cm-I, and a OH distance in the free proton-donor molecule of 0.97 X cm. The O...N distances have been evaluated by the relations of Nakam~to.~' Table X lists the computed koH and k , values. For a series of complexes, these values are ordered according to the acidity of the phenol derivative. It must be pointed out that the k, values calculated by the function of Lippincott and Schroeder are intermediate between those computed by using the reduced mass of the OH.* .N moiety and those calculated by taking into account the whole mass of the base and acid molecules. For the complex phenol-pyrimidine, for example, one should obtain k, values of 6.84 and 38.53 N m-l, respectively. This is qualitatively in agreement with the full force-field treatment of phenol-pyridine complexes which has shown that the bases are represented by point masses taken as 70% of the full molecular weight.38
Acknowledgment. This work was supported by the Catholic University of Leuven. O.K. is indebted to the ABOS (Belgium) for a fellowship. Registry No. 3,4-Dimethylphenol,95-65-8; 4-methoxyphenol, 15076-5; phenol, 108-95-2;4-fluorophenol, 371-41-5; 4-chlorophenol, 10648-9; 4-bromophenol, 106-41-2;3-fluorophenol, 372-20-3;3-chlorophenol, 108-43-0; 3-bromophenol, 591-20-8; 3,4-dichlorophenol, 95-77-2; 3nitrophenol, 554-84-7; 3,5-dichlorophenol, 591-35-5; pyridazine, 28980-5; pyrimidine, 289-95-2; 2-aminopyrimidine, 109-12-6; pyrazine, 290-37-9; 3,4,5-trichlorophenol,609-19-8. (35) Reyntjens-Van Damme, D.; Zeegers-Huyskens, Th. Adu. Mol. Relaxation Interact. Processes 1980, 16, 15. (36) Lippincott, E. R.; Schroeder, R.J. Chem. Phys. 1955, 23, 1099; J . Am. Chem. SOC.1956,78,5171; J. Phys. Chem. 1957,61,92;J. Chem. Phys.
__.
19S7, 26 I611. .
(37) Nakamoto, K.; Margoshes, M.; Rundle, R. E. J . A m . Chem. SOC. 1955, 77,6480. (38) Dean, R. L.; Wood, J. L. J . Mol. Struct. 1975, 26, 197.
