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Mid- and near-infrared study of hydrogen bond complexes between phenols and N,N-dimethylformamide
Vibrational Spectroscopy 18 Ž1998. 41–49
Mid- and near-infrared study of hydrogen bond complexes between phenols and N, N-dimethylformamide Z. Pawelka a , Th. Zeegers-Huyskens
b,)
a b
Faculty of Chemistry, UniÕersity of Wroclaw, 14 Joliot-Curie Street, 50383 Wroclaw, Poland Department of Chemistry, UniÕersity of LeuÕen, 200F Celestijnenlaan, 3001 HeÕerlee, Belgium Received 27 May 1998; revised 21 August 1998; accepted 24 August 1998
Abstract The mid- and near-infrared spectra of hydrogen bond complexes between phenol derivatives and N, N-dimethylformamide are investigated. The enthalpies of complex formation determined in carbon tetrachloride range from 23 to 26.6 kJ moly1. Some spectroscopic features observed in the mid- and near-infrared region are discussed. The first overtone of the n ŽOH . . . O. vibration is very broad and characterized by several submaxima which are tentatively assigned to a coupling between high and low frequency modes. The anharmonicities of the n ŽOH . . . O. vibration determined from the experimental frequencies observed in the mid-infrared and near-infrared region range between 117 and 150 cmy1. The correlation between the enthalpies and experimental or harmonic frequency shifts is discussed. Comparison with complexes involving the same proton donors and N-methyldiacetamide shows that the anharmonicity roughly increases with the hydrogen bond strength. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen bonds; Phenols; N, N-dimethylformamide; Thermodynamic data; Anharmonicities
1. Introduction Anharmonicity is a fundamental quantity for the understanding of the nature of the hydrogen bonding, and to gain insight into the nature of hydrogen bonding, it is essential to obtain information on the shape of the potential which is intimately related to the anharmonicity w1x. The anharmonicity can be calculated by ab initio methods only for simple hydrogen-bonded systems such as the wH2NH . . . NH2xy w2x or wHOH . . . OHxy w3x ones, but can be obtained experimentally by measuring overtones in ) Corresponding author. Tel.: q32-16-327477; Fax: q32-16327991
the infrared spectrum. However, as pointed out recently by Sandorfy w4x, the near-infrared region where the overtones of the OH, NH stretching vibrations are observed, is seldom used by the researchers and these last two decades, very few experimental nearinfrared data on hydrogen-bonded systems and on the effect of hydrogen bonding on the anharmonicity of the vibrations have been published. The reason is perhaps that overtones of hydrogen-bonded species have a weak intensity and that the near-infrared spectra of polyatomic molecules are generally characterized by numerous bands originating not only from overtones, but also from combinations or simultaneous transitions, which makes the assignment of the absorptions particularly difficult w5,6x.
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 3 8 - 1
42
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
Studies performed on hydrogen bonds of medium strength have shown that the anharmonicity of the OH or NH stretching vibration generally increases by hydrogen bond formation w7–12x. From these data relative to different proton donors and proton acceptors, solvents and temperatures, no general conclusions about the variation of the anharmonicity with the hydrogen bond strength can be drawn. In recent works w13,14x, we have investigated the anharmonicity in hydrogen bonds where the proton donors are phenols. The phenols characterized by different acidities allow the variation of the hydrogen bond strength in closely related systems. In the present work, the complexes involving phenol derivatives and N, N-dimethylformamide ŽDMF.
are investigated in the mid-infrared and near-infrared range. The anharmonicity of the n ŽOH . . . O. vibration is discussed as a function of the strength of the hydrogen bond and compared with recent results on complexes involving the same proton donors and N-methyldiacetamide w14x.
the proton donors ranged between 5 and 10.10y3 mol dmy3 in order to avoid self-association. The base was always in excess. The error on the determination on K is 5%. The cell thickness was 0.3 cm. The measurements have been carried out in carbon tetrachloride at the temperatures of 298 and 323 K. The spectra in the mid- and near-infrared region have been recorded on the Bruker 66-FT-IR spectrometer at a resolution of 2 cmy1 Ž16 scans.. In the mid-infrared region, the spectrometer was equipped with a Globar source, a KBr beamsplitter and a DTGS detector. The path length of the KBr cells used for the mid-infrared range was 0.3 cm. In the near-infrared region, a Tungsten lamp was used as source, a CaF2 beamsplitter and a cooled InSb detector. The path length of the quartz cell was 5 cm. The near-infrared spectra were taken at room temperature Ž296 K.. The overtones of the n ŽCH. vibrations very often overlap with the broad absorption of the hydrogen bond complexes. DMF-d7 has been used for the study of the near-infrared spectra. DMF and DMF-d7 from Acros have been used without further purification. The phenols from Aldrich or Acros have been crystallized from petroleum ether. Carbon tetrachloride ŽRiedel De ˚ .. Haen. was dried over molecular sieves Ž4 A
3. Experimental results and discussion 3.1. Thermodynamic data and mid-infrared spectra of the complexes inÕolÕing phenols and DMF
2. Experimental
Table 1 contains the formation constants Ž K . determined at 298 and 323 K of the complexes
The infrared spectra for the determination of the equilibrium constants have been recorded on the Perkin-Elmer 883. The equilibrium constants defined as
Table 1 Thermodynamic data for the complexes involving phenols and DMF in carbon tetrachloride Phenols
K s CabrCa Cb s Ž Fa y Ca . rCa Ž Fb y Cab . , where Cab, Ca, and Cb are the molar concentrations of the complex, of free phenol and of free base, and Fa and Fb are the analytical concentrations of the proton donor and proton acceptor. The concentration Ca has been determined from the absorbance of the n ŽOH. stretching vibration of the free phenol derivative, lying at 3610–3600 cmy1 . The concentration of
p Ka
K 298 K K 323 K y DH Ž1 moly1 . Ž1 moly1 . ŽkJ moly1 .
4-OCH 3 phenol 10.21 47 phenol 9.95 59.3 4-Cl phenol 9.42 152 3-Br phenol 9.03 194 3,4-diCl 2 phenol 8.58 387 3,5-diCl 2 phenol 8.18 520 a
23 28.1 71 88.5 172 228
22.9 23.9 a 24.4 25.1 26 26.5
The y DH values cited in Refs. w15,16x are 25.5 and 25.9 kJ moly1 , respectively.
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
between phenol derivatives and DMF, the enthalpies of complex formation ŽyDH. along with the p K a of the phenols. Linear correlations between the logarithms of the formation constants and the p K a of the phenols are obtained. log K 298 K s 7.15 y 0.536 p K a Ž r s 0.993 .
Ž 1.
log K 323 K s 6.57 y 0.510 p K a Ž r s 0.992 .
Ž 2.
The complexes are of medium strength, the enthalpies of complex formation ranging from 22.9 to 26.5 kJ moly1 . Table 2 contains the frequencies of the n ŽOH. vibration observed in the free phenols and in the complexes and the frequency shifts D n 01 ŽOH. observed in the fundamental region. One example of spectrum is reproduced in Fig. 1. This spectrum taken in great excess of base does not show the free n 01 ŽOH. band. In this small range, the correlation between the experimental frequency shifts and the enthalpies of complex formation is linear. yDH s 14.53 q 0.031D n 01 Ž OH .
exp
Ž r s 0.983 . Ž 3.
The difference observed between the D n ŽOH. values for the complexes of DMF and DMF-d7 are of the order of magnitude of the experimental errors on D n ŽOH. Ž"5 cmy1 .. The vibrational spectrum of DMF and DMD-d7 have been discussed in several works w17–20x and we will briefly discuss the spectra of the complexes in the mid-infrared region. The n C5O band observed at 1686 cmy1 in free DMF is shifted to 1677 cmy1 in the phenol complex and to 1672 cmy1 in the 3,5-dichlorophenol complex. These shifts are very similar to those observed in N, N-dimethylacetamide complexed with the same proton donors
w21x. In free DMF-d7 , two bands are observed at 1699 and 1665 cmy1 . The intensity of both absorptions decreases on complex formation and a new band is observed to the low frequency side, at 1653 cmy1 in the phenol complex and at 1646 cmy1 in the 3,5-dichlorophenol complex. The high frequency band observed in free DMF-d7 at 1699 cmy1 was not mentioned in Ref. w17x, but was assigned in Ref. w19x to a mode involving the n C5O, d C5O and n CD vibrations and the low frequency component at 1665 cmy1 to a mode involving mainly the n C5O vibration. For all the complexes studied in the present work, the intensity of the two bands decreases on complex formation and this suggests that the n C5O stretching mode is involved in both absorptions in agreement with the assignment of Ref. w19x. The n ŽC-N. vibration observed at 1384 cmy1 and the n as ŽCX NCX . mode observed at 1267 cmy1 in free DMF-d7 are also weakly perturbed by complex formation. They are observed at 1388 and 1271 cmy1 in the 3,5-dichlorophenol complex. The phenol vibrations are also weakly perturbed by complex formation. 3.2. Near-infrared spectra of the phenols–DMF-d7 complexes It is useful to remember here that the second-order perturbation theory allows one to compute the mechanical anharmonicity X from the experimental frequencies of the fundamental Ž n 01 ., first Ž n 02 . or second overtone Ž n 03 .. X s n 01 y n 02 r2 s n 02 r2 y n 03 r3
Phenol
n 01 ŽOH. f
n 01 ŽOH . . . O. exp a
D n 01 ŽOH.
4-OCH 3 -phenol phenol 4-Cl phenol 3-Br phenol 3,4-diCl 2 phenol 3,5-diCl 2 phenol
3613 3611 3609 3605 3604 3601
3340 3314 3276 3270 3236 3220
268 297 333 335 368 381
a
Error on the absorption maximum s"5 cmy1 .
Ž 4.
e.
The harmonic frequency Ž n can be computed by the relation
n e s n 01 q 2 X Table 2 Experimental data Žcmy1 . for the n 01 ŽOH. vibration in free phenols and their complexes with DMF
43
Ž 5.
The coupling constant Ž X 12 . between two different vibrations characterized by fundamental frequencies n 101 and n 201 can be calculated from the expression X 12 s n 02 y Ž n 101 q n 201 . .
Ž 6.
The second-order perturbation theory can be considered as valid for weakly hydrogen-bonded systems, but has been questioned for strongly interacting systems w22x.
44
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
Fig. 1. Mid-IR spectrum Ž4000–500 cmy1 . of a solution of 3,5-dichlorophenol Ž c s 0.06 mol dmy3 . and DMF-d7 Ž c s 0.12 mol dmy3 .. Solvents carbon tetrachloride, Path length s 0.2 cm.
3.2.1. Assignment of the absorptions in the near-infrared region Primary and secondary aliphatic amides give rise to several absorptions in the near-infrared region w23–25x. These absorptions have been assigned to combinations of the 2 n C5O or n NH vibrations with the amide II or amide III vibrations. In the near-infrared spectrum of DMF-d7 , only very weak absorptions are observed between 5500 and 4500 cmy1 . The bands at 4967 and 4927 cmy1 can be assigned to the second overtone of the n C5O modes observed at 1699 and 1665 cmy1 in the fundamental region. This assignment is strengthened by the fact that only one absorption at 5016 cmy1 is observed in the spectrum of DMF. Further, the most intense absorptions observed at 4449, 4407, 4238, and 4172 cmy1 are due to the overtones of the n C-D vibrations. Owing to the overlapping with the strong absorptions of the phenols, the perturbation of the DMF-d7 modes resulting from complex formation could not be observed. This is shown in Fig. 2 which reproduces the near-infrared spectrum of 4-Cl phenol
and the 4-Cl phenol–DMF-d7 complex. In free 4-Cl phenol, the n 02 ŽOH. vibration is observed at 7048 cmy1 and the absorptions observed at 5203, 5032, 4926, 4869, and 4781 cmy1 have been assigned to combinations involving the n ŽOH. vibration and internal modes Ž n 8 a, n 19b, n 14, n C-O and d OH. of 4-Cl phenol w26x. In the DMF-d7 complex, the intensity of these absorptions decreases and this confirms our previous assignment. Very recently, a very similar assignment of jet-cooled phenol has been proposed w27x. The n 02 ŽOH . . . O. vibration is observed at 6270 cmy1 and the anharmonicity of this vibration will be discussed in the next section. For all the complexes, an overall intensity increase of the absorption in the 5750–4700 cmy1 region is observed. This is clearly shown in Fig. 2 and in Fig. 3 which reproduces the spectrum between 5300 and 4700 cmy1 of phenol–DMF-d7 solutions taken at different DMF-d7 concentrations. In this complex also, the absorptions observed at 5209, 5080, 4949, 4867, and 4783 cmy1 in free phenol decrease in the DMF-d7 complex and new broad bands are observed at about
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
45
Fig. 2. Near-IR spectrum Ž8000–4000 cmy1 . of ŽA. 4-Cl phenol Ž c s 0.06 mol dmy3 .; ŽB. 4-Cl phenol Ž c s 0.06 mol dmy3 . and DMF-d7 Ž c s 0.18 mol dmy3 ., Solvents carbon tetrachoride, Path lengths 5 cm; ŽC. difference spectrum between B and A.
Fig. 3. Near-IR spectrum Ž5300–4700 cmy1 . of the complex between phenol and DMF-d7 . Concentration of phenols 0.04 mol dmy3 . Concentration of DMF-d7 : Ž1. s 0, Ž2. s 0.036 mol dmy3 , Ž3. s 0.054 mol dmy3 , Ž4. s 0.077 mol dmy3 , Ž5. s 0.123 mol dmy3 .
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
46
5440, 5275, 5145, 5010, and 4860 cmy1 . The intensity of these bands is very weak. The extinction coefficient of the band at 5010 cmy1 estimated from the absorbance and the concentration of complexes is about 0.2 moly1 cmy1 and is much smaller than the extinction coefficient of the free phenol absorption at 4949 cmy1 which is about 1.25 moly1 cmy1 . No submaxima could be observed between 6100 and 5750 cmy1 owing to overlapping with the 2 n ŽCH. vibrations of the phenols. The broad absorption, somewhat similar to the ‘continuum’ observed in the mid-infrared region w28x, but of much lower intensity, is intriguing and cannot be explained by proton transfer in the second vibrational excited state. The complexes studied in this work are characterized by ˚ and the barrier RŽO . . . O. distances of about 2.75 A height of proton transfer is in this case higher than 10,000 cmy1 w29x. In order to provide some explanations on the observed sub-maxima, the frequencies of some relevant absorptions observed in the mid-infrared region of free phenol and the phenol–DMF-d7 complex are indicated in Table 3. As shown by these data, the complex absorptions cannot be assigned to combinations involving the n ŽOH . . . O. and n 8 a, n 19b, n 14 or n C-O vibrations. In the spectrum of self-associated aliphatic alcohols, the broad absorption observed between 4600 and 5000 cmy1 has been assigned to the n ŽOH . . . O. q d ŽOH. combination w30x. In the present complex, this combination should give an absorption around 4550 cmy1 which
Table 3 Mid-infrared and near-infrared data Žcmy1 . observed in free phenol and the phenol-DMF-d7 complex Free phenol
Phenol–DMF-d7 complex
Assignment
3611 1608 1597 1499 1470 1342 1255 1178 310
3314 1606 1594 1501 1473 1345a 1267 a 1231 690 170 b
n ŽOH. n 8a n 8b n 19 a n 19b n 14 n C-O d ŽOH. g ŽOH. ns ŽH . . . O.
a b
Overlapping with a DMF-d7 absorption. From Ref. w30x.
was not observed. A simultaneous excitation w7,30– 32x involving the n ŽOH . . . O. and n ŽC5O. vibrations should result in an absorption at 4967 cmy1 and must also be ruled out. The distances between the absorptions observed between 5450 and 4860 cmy1 is between 130 and 160 cmy1 . The intermolecular stretching vibration Ž ns . which is observed at 150 cmy1 in the complex phenol-N-methylacetamide w33x must be of the same of the same order of magnitude for the present complex, the strength of the interaction and the reduced mass of the interacting molecules being about the same w34x. Combination between the high and low frequency modes 2 n ŽOH . . . O. " nns is perhaps at the origin of the broadness and the band shape. To the best of our knowledge, the combination between the first overtone of the stretching vibration and the intermolecular mode has been observed only for the dimethylether–hydrogen fluoride complex in the gas phase w35x and the Millen satellite bands seem to have a higher intensity in the overtone region than in the fundamental one w36x. In the present complexes also, some fine structure of the n ŽOH . . . O. stretching vibration is also observed in the fundamental region, between 3000 and 2500 cmy1 . Sheppard w37x has pointed out that the proton will be on the average, nearer to the base in the Õ s 1 state than in the Õ s 0 state and as a consequence, the hydrogen bond will be stronger in the first excited vibrational state than in the fundamental one. These effects are expected to be more pronounced in the second vibrational state and the coupling between the high and low frequency modes more effective. Thus, in the present complexes, the first overtone of the n ŽOH . . . O. vibration is very broad and shows a pronounced asymmetry at the low frequency side. The subbands are tentatively assigned to combinations 2 n ŽOH . . . O. y nns . Measurements at low temperature where the hot bands disappear would confirm our interpretation. As suggested by one of the referees, the band substructure could also be explained by the well-known ‘Evans Holes’ reported many times for strong hydrogen bonds. This explanation cannot be ruled out, but seems less probable because the complexes studied in this work are not very strong and in the fundamental region, the n 01 ŽOH. band does not show a sudden interruption. However, in both interpretations, the anharmonicity
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49 Table 4 Near-infrared data for free phenols and their complexes with DMF-d7 . Phenol 4-OCH 3 phenol phenol 4-Cl phenol 3-Br phenol 3,4-diCl 2 phenol 3,5-diCl 2 phenol
n 02 ŽOH. f 7057 7052 7048 7041 7040 7033
n 02 ŽOH . . . O. a
Xb
6445 6380 6270 6260 6185 6140
117 124 141 140 144 150
D n ŽOH. e 199 219 221 225 250 251
a
Experimental error on the absorption maximum"15 cmy1 . The experimental error on the anharmonicities estimated from the errors on the experimental frequencies in the mid- and near-infrared region is "12 cmy1 . Anharmonicity of the n ŽOH . . . O. vibration Ždata in cmy1 . and harmonic frequency shift.
b
is an important factor in the band-shaping mechanism. 3.2.2. Anharmonicity of the n (OH . . . O) Õibration and hydrogen bond strength In a solution of 4-Cl phenol and DMF-d7 , the intensity of the free n 02 ŽOH.absorption at 7048 cmy1 decreases and a new band assigned to the n 02 ŽOH. vibration in the complex is observed at 6270 cmy1 . The maximum of this absorption has been determined by subtracting the spectrum of phenol at the same concentration to the spectrum of the ternary solution. The error on the position of the n 02 ŽOH . . . O. absorption is "15 cmy1 . Table 4 lists the experimental n 02 ŽOH. values in free phenols and their complexes with DMF-d7 , the
Fig. 4. D n ŽOH. Žcmy1 . as a function of y DH ŽkJ moly1 . experimental frequency shifts or harmonic frequency shifts.
47
Table 5 Enthalpies of complex formation and anharmonicities of the n ŽOH . . . O. vibration for complexes between phenols and Nmethyldiacetamide Ždata from Ref. w14x. Phenol
y DH ŽkJ moly1 .
X Žcmy1 .
3,4-diCH 3 phenol 4-CH 3 phenol phenol 4-Cl phenol 4-Br phenol 3-Br phenol 3,4-diCl 2 phenol 3,5-diCl 2 phenol
19.3 20.6 21.4 21.6 23.4 23.5 23.7 24
80 83 80 85 85 95 102 106
anharmonicity constants along with the harmonic frequency shifts Ž D n e ŽOH... The harmonic shifts are computed from Eq. Ž5. taking a value of 85 cmy1 for the anharmonicity of the n ŽOH. vibration of the free phenols w26x. Inspection of the results of Table 2 shows that the anharmonicities increase with the hydrogen bond strength. The correlation between the enthalpies of complex formation and the harmonic frequency shifts can be written as f
yD H s a q b Ž n 01 Ž OH . y n 01 Ž OH . . . O . q2 X c y 2 X f ,
exp
Ž 7.
where X f and X c refer to the anharmonicities of the n ŽOH. vibration in the free and complexed molecules.
Fig. 5. X Žcmy1 . as a function of y DH ŽkJ moly1 . for complexes involving phenols and DMF-d7 Žthis work. and N-methylsuccinimide ŽRef. w14x..
48
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
In the present case, the validity of the Badger– Bauer correlation can be explained by the fact that, as indicated by the data of Tables 1 and 4, the increase of anharmonicity resulting from hydrogen bonding is roughly proportional to the enthalpy of complex formation. As suggested by Sandorfy w1x, the hydrogen bond frequency shift reflects not only the change in the equilibrium distance that occurs when the hydrogen bond is formed, but also the accompanying changes in the values of the anharmonicity constants. The validity of the Badger–Bauer correlation thus implies that the change in anharmonicity is roughly proportional to the change in equilibrium distance. In the present case, the Badger–Bauer correlation can be written as e
yD H s 10.50 q 0.063D n Ž OH . Ž r s 0.978 . .
Ž 8. The correlation between the enthalpies of complex formation and the experimental or harmonic frequency shifts is illustrated in Fig. 4. Table 5 lists the values of the anharmonicities and the enthalpies of complex formation obtained recently for the complexes between N-methyldiacetamide CH 3 CŽ5O.NCH 3 CŽ5.OCH 3 and phenol derivatives w14x. These complexes belong to the same family and are somewhat weaker than the DMF complexes. Fig. 5 shows that the increase of the anharmonicity with the hydrogen bond strength can be extended in a broader domain. The scatter of the points observed in Fig. 5 is due to the experimental errors on the anharmonicity Ž"12 cmy1 . and on the enthalpies on complex formation Ž"1 kJ moly1 .. Also, as previously discussed, the second-order perturbation theory is perhaps not entirely valid for these hydrogen bonds of medium strength. For the weakest hydrogen bonds, the anharmonicity is of the same order of magnitude as that of the free n ŽOH. vibration. This is agreement with the experimental data of Sandorfy on weak hydrogen bonds existing as for instance in self-associated aliphatic amines. For these weak hydrogen bonds, the anharmonicity of the n ŽNH. vibration is nearly the same in the monomers and self-associated species w12x. Our results are in qualitative agreement with theoretical data on OH . . . O systems w3x. The anharmonicity of
the n ŽOH . . . O. vibration has been computed as a function of the RŽO . . . O. distance. For weak and medium hydrogen bonds, the anharmonicity increases slightly with decreasing RŽO . . . O. distances. Acknowledgements The authors thank the Fund of Scientific Research-Vlaanderen for financial support. This work has been done in the frame of the bilateral cooperation between Poland and Flanders. Z.P. thanks the Flemish Community for a post-doctoral fellowship. References w1x C. Sandorfy, The hydrogen bond, Recent Developments in Theory and Experiments: II. Structure and Spectroscopy, in: P. Schuster, G. Zundel, C. Sandorfy ŽEds.., North-Holland, Amsterdam, 1976, p. 645. w2x W.A.P. Luck, T. Wess, J. Mol. Struct. 270 Ž1992. 229. w3x W.A.P. Luck, T. Wess, Can. J. Chem. 69 Ž1991. 1919. w4x C. Sandorfy, Bull. Acad. Pol. Sci. 43 Ž1995. 7. w5x C. Sandorfy, Top. Curr. Chem. 120 Ž1984. 42. w6x W.A.P. Luck, Intermolecular forces, An Introduction to Modern Methods and Results, in: P. Huyskens, W.A.P. Luck, Th. Zeegers-Huyskens ŽEds.., Springer-Verlag, Berlin, 1991, p. 157. w7x M. Asselin, C. Sandorfy, J. Chem. Phys. 52 Ž1970. 6130. w8x M. Asselin, C. Sandorfy, Chem. Phys. Lett. 8 Ž1971. 601. w9x C. Bourderon, J.J. Peron, C. Sandorfy, J. Phys. Chem. 76 Ž1972. 864. w10x M.C. Bernard-Houplain, C. Sandorfy, Can. J. Chem. 51 Ž1973. 1075. w11x M.C. Bernard-Houplain, C. Sandorfy, Can. J. Chem. 52 Ž1973. 3640. w12x M.C. Bernard-Houplain, C. Sandorfy, J. Chem. Phys. 56 Ž1972. 3412. w13x M. Rospenk, Th. Zeegers-Huyskens, J. Phys. Chem. 101 Ž1997. 8428. w14x N. Leroux, C. Samyn, Th. Zeegers-Huyskens, J. Mol. Struct., in press. w15x M.D. Joesten, R.S. Drago, J. Am. Chem. Soc. 84 Ž1962. 3817. w16x H. Fritsche, Ber. Bunsenges. Physik. Chem. 68 Ž1964. 459. w17x G. Durgaprasad, D.N. Santhyanarayana, C.C. Patel, Bull. Chem. Soc. Japan 44 Ž1971. 316. w18x E.W. Randall, C.M. Yoder, J.J. Zuckerman, Inorg. Chem. 5 Ž1966. 2240. w19x D. Steele, A. Quatermain, Spectrochim. Acta 43A Ž1987. 781. w20x X. Zhou, J.A. Krauser, D.R. Tate, A.S. Van Buren, J.A. Clark, P.R. Moody, R. Liu, J. Phys. Chem. 100 Ž1996. 16822.
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49 w21x C. Dorval, Th. Zeegers-Huyskens, Spectrochim. Acta 29A Ž1973. 1805. w22x A. Foldes, C. Sandorfy, J. Mol. Spectrosc. 20 Ž1966. 262. w23x S.E. Krikorian, M. Marpour, Spectrochim. Acta 29A Ž1973. 1233. w24x S.E. Krikorian, Spectrochim. Acta 37A Ž1981. 745. w25x P.J. McNeilly, T.A. Andrea, S.E. Krikorian, Spectrochim. Acta 48A Ž1992. 1437. w26x M. Rospenk, N. Leroux, Th. Zeegers-Huyskens, J. Mol. Spectry. 183 Ž1997. 245. w27x S.-I. Ishiuchi, H. Shitomi, K. Takazawa, M. Fujii, Chem. Phys. Lett. 283 Ž1998. 243. w28x G. Zundel, Ref. w1x, p. 687.
w29x w30x w31x w32x w33x w34x w35x w36x w37x
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T. Saitoh, K. Mori, R. Itoh, Chem. Phys. 60 Ž1981. 161. M. Asselin, C. Sandorfy, Can. J. Chem. 49 Ž1971. 1539. A. Burneau, J. Corset, J. Chem. Phys. 56 Ž1972. 662. A. Burneau, Spectrochim. Acta 45A Ž1989. 1305. C. Dorval, Th. Zeegers-Huyskens, Spectrochim. Acta 30A Ž1974. 1757. E. Knozinger, O. Schrems, in: J.R. Durig ŽEd.., Vibrational ¨ Spectra and Structure, Vol. 16, Elsevier, 1987. J.W. Bevan, B. Martineau, C. Sandorfy, Can. J. Chem. 57 Ž1979. 1341. J. Arnold, D.J. Millen, J. Chem. Soc., 1965, 503. N. Sheppard, in: D. Hadzi ŽEd.., Hydrogen Bonding, Pergamon, London, 1959, p. 85.
Mid- and near-infrared study of hydrogen bond complexes between phenols and N, N-dimethylformamide Z. Pawelka a , Th. Zeegers-Huyskens
b,)
a b
Faculty of Chemistry, UniÕersity of Wroclaw, 14 Joliot-Curie Street, 50383 Wroclaw, Poland Department of Chemistry, UniÕersity of LeuÕen, 200F Celestijnenlaan, 3001 HeÕerlee, Belgium Received 27 May 1998; revised 21 August 1998; accepted 24 August 1998
Abstract The mid- and near-infrared spectra of hydrogen bond complexes between phenol derivatives and N, N-dimethylformamide are investigated. The enthalpies of complex formation determined in carbon tetrachloride range from 23 to 26.6 kJ moly1. Some spectroscopic features observed in the mid- and near-infrared region are discussed. The first overtone of the n ŽOH . . . O. vibration is very broad and characterized by several submaxima which are tentatively assigned to a coupling between high and low frequency modes. The anharmonicities of the n ŽOH . . . O. vibration determined from the experimental frequencies observed in the mid-infrared and near-infrared region range between 117 and 150 cmy1. The correlation between the enthalpies and experimental or harmonic frequency shifts is discussed. Comparison with complexes involving the same proton donors and N-methyldiacetamide shows that the anharmonicity roughly increases with the hydrogen bond strength. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen bonds; Phenols; N, N-dimethylformamide; Thermodynamic data; Anharmonicities
1. Introduction Anharmonicity is a fundamental quantity for the understanding of the nature of the hydrogen bonding, and to gain insight into the nature of hydrogen bonding, it is essential to obtain information on the shape of the potential which is intimately related to the anharmonicity w1x. The anharmonicity can be calculated by ab initio methods only for simple hydrogen-bonded systems such as the wH2NH . . . NH2xy w2x or wHOH . . . OHxy w3x ones, but can be obtained experimentally by measuring overtones in ) Corresponding author. Tel.: q32-16-327477; Fax: q32-16327991
the infrared spectrum. However, as pointed out recently by Sandorfy w4x, the near-infrared region where the overtones of the OH, NH stretching vibrations are observed, is seldom used by the researchers and these last two decades, very few experimental nearinfrared data on hydrogen-bonded systems and on the effect of hydrogen bonding on the anharmonicity of the vibrations have been published. The reason is perhaps that overtones of hydrogen-bonded species have a weak intensity and that the near-infrared spectra of polyatomic molecules are generally characterized by numerous bands originating not only from overtones, but also from combinations or simultaneous transitions, which makes the assignment of the absorptions particularly difficult w5,6x.
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 3 8 - 1
42
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
Studies performed on hydrogen bonds of medium strength have shown that the anharmonicity of the OH or NH stretching vibration generally increases by hydrogen bond formation w7–12x. From these data relative to different proton donors and proton acceptors, solvents and temperatures, no general conclusions about the variation of the anharmonicity with the hydrogen bond strength can be drawn. In recent works w13,14x, we have investigated the anharmonicity in hydrogen bonds where the proton donors are phenols. The phenols characterized by different acidities allow the variation of the hydrogen bond strength in closely related systems. In the present work, the complexes involving phenol derivatives and N, N-dimethylformamide ŽDMF.
are investigated in the mid-infrared and near-infrared range. The anharmonicity of the n ŽOH . . . O. vibration is discussed as a function of the strength of the hydrogen bond and compared with recent results on complexes involving the same proton donors and N-methyldiacetamide w14x.
the proton donors ranged between 5 and 10.10y3 mol dmy3 in order to avoid self-association. The base was always in excess. The error on the determination on K is 5%. The cell thickness was 0.3 cm. The measurements have been carried out in carbon tetrachloride at the temperatures of 298 and 323 K. The spectra in the mid- and near-infrared region have been recorded on the Bruker 66-FT-IR spectrometer at a resolution of 2 cmy1 Ž16 scans.. In the mid-infrared region, the spectrometer was equipped with a Globar source, a KBr beamsplitter and a DTGS detector. The path length of the KBr cells used for the mid-infrared range was 0.3 cm. In the near-infrared region, a Tungsten lamp was used as source, a CaF2 beamsplitter and a cooled InSb detector. The path length of the quartz cell was 5 cm. The near-infrared spectra were taken at room temperature Ž296 K.. The overtones of the n ŽCH. vibrations very often overlap with the broad absorption of the hydrogen bond complexes. DMF-d7 has been used for the study of the near-infrared spectra. DMF and DMF-d7 from Acros have been used without further purification. The phenols from Aldrich or Acros have been crystallized from petroleum ether. Carbon tetrachloride ŽRiedel De ˚ .. Haen. was dried over molecular sieves Ž4 A
3. Experimental results and discussion 3.1. Thermodynamic data and mid-infrared spectra of the complexes inÕolÕing phenols and DMF
2. Experimental
Table 1 contains the formation constants Ž K . determined at 298 and 323 K of the complexes
The infrared spectra for the determination of the equilibrium constants have been recorded on the Perkin-Elmer 883. The equilibrium constants defined as
Table 1 Thermodynamic data for the complexes involving phenols and DMF in carbon tetrachloride Phenols
K s CabrCa Cb s Ž Fa y Ca . rCa Ž Fb y Cab . , where Cab, Ca, and Cb are the molar concentrations of the complex, of free phenol and of free base, and Fa and Fb are the analytical concentrations of the proton donor and proton acceptor. The concentration Ca has been determined from the absorbance of the n ŽOH. stretching vibration of the free phenol derivative, lying at 3610–3600 cmy1 . The concentration of
p Ka
K 298 K K 323 K y DH Ž1 moly1 . Ž1 moly1 . ŽkJ moly1 .
4-OCH 3 phenol 10.21 47 phenol 9.95 59.3 4-Cl phenol 9.42 152 3-Br phenol 9.03 194 3,4-diCl 2 phenol 8.58 387 3,5-diCl 2 phenol 8.18 520 a
23 28.1 71 88.5 172 228
22.9 23.9 a 24.4 25.1 26 26.5
The y DH values cited in Refs. w15,16x are 25.5 and 25.9 kJ moly1 , respectively.
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
between phenol derivatives and DMF, the enthalpies of complex formation ŽyDH. along with the p K a of the phenols. Linear correlations between the logarithms of the formation constants and the p K a of the phenols are obtained. log K 298 K s 7.15 y 0.536 p K a Ž r s 0.993 .
Ž 1.
log K 323 K s 6.57 y 0.510 p K a Ž r s 0.992 .
Ž 2.
The complexes are of medium strength, the enthalpies of complex formation ranging from 22.9 to 26.5 kJ moly1 . Table 2 contains the frequencies of the n ŽOH. vibration observed in the free phenols and in the complexes and the frequency shifts D n 01 ŽOH. observed in the fundamental region. One example of spectrum is reproduced in Fig. 1. This spectrum taken in great excess of base does not show the free n 01 ŽOH. band. In this small range, the correlation between the experimental frequency shifts and the enthalpies of complex formation is linear. yDH s 14.53 q 0.031D n 01 Ž OH .
exp
Ž r s 0.983 . Ž 3.
The difference observed between the D n ŽOH. values for the complexes of DMF and DMF-d7 are of the order of magnitude of the experimental errors on D n ŽOH. Ž"5 cmy1 .. The vibrational spectrum of DMF and DMD-d7 have been discussed in several works w17–20x and we will briefly discuss the spectra of the complexes in the mid-infrared region. The n C5O band observed at 1686 cmy1 in free DMF is shifted to 1677 cmy1 in the phenol complex and to 1672 cmy1 in the 3,5-dichlorophenol complex. These shifts are very similar to those observed in N, N-dimethylacetamide complexed with the same proton donors
w21x. In free DMF-d7 , two bands are observed at 1699 and 1665 cmy1 . The intensity of both absorptions decreases on complex formation and a new band is observed to the low frequency side, at 1653 cmy1 in the phenol complex and at 1646 cmy1 in the 3,5-dichlorophenol complex. The high frequency band observed in free DMF-d7 at 1699 cmy1 was not mentioned in Ref. w17x, but was assigned in Ref. w19x to a mode involving the n C5O, d C5O and n CD vibrations and the low frequency component at 1665 cmy1 to a mode involving mainly the n C5O vibration. For all the complexes studied in the present work, the intensity of the two bands decreases on complex formation and this suggests that the n C5O stretching mode is involved in both absorptions in agreement with the assignment of Ref. w19x. The n ŽC-N. vibration observed at 1384 cmy1 and the n as ŽCX NCX . mode observed at 1267 cmy1 in free DMF-d7 are also weakly perturbed by complex formation. They are observed at 1388 and 1271 cmy1 in the 3,5-dichlorophenol complex. The phenol vibrations are also weakly perturbed by complex formation. 3.2. Near-infrared spectra of the phenols–DMF-d7 complexes It is useful to remember here that the second-order perturbation theory allows one to compute the mechanical anharmonicity X from the experimental frequencies of the fundamental Ž n 01 ., first Ž n 02 . or second overtone Ž n 03 .. X s n 01 y n 02 r2 s n 02 r2 y n 03 r3
Phenol
n 01 ŽOH. f
n 01 ŽOH . . . O. exp a
D n 01 ŽOH.
4-OCH 3 -phenol phenol 4-Cl phenol 3-Br phenol 3,4-diCl 2 phenol 3,5-diCl 2 phenol
3613 3611 3609 3605 3604 3601
3340 3314 3276 3270 3236 3220
268 297 333 335 368 381
a
Error on the absorption maximum s"5 cmy1 .
Ž 4.
e.
The harmonic frequency Ž n can be computed by the relation
n e s n 01 q 2 X Table 2 Experimental data Žcmy1 . for the n 01 ŽOH. vibration in free phenols and their complexes with DMF
43
Ž 5.
The coupling constant Ž X 12 . between two different vibrations characterized by fundamental frequencies n 101 and n 201 can be calculated from the expression X 12 s n 02 y Ž n 101 q n 201 . .
Ž 6.
The second-order perturbation theory can be considered as valid for weakly hydrogen-bonded systems, but has been questioned for strongly interacting systems w22x.
44
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
Fig. 1. Mid-IR spectrum Ž4000–500 cmy1 . of a solution of 3,5-dichlorophenol Ž c s 0.06 mol dmy3 . and DMF-d7 Ž c s 0.12 mol dmy3 .. Solvents carbon tetrachloride, Path length s 0.2 cm.
3.2.1. Assignment of the absorptions in the near-infrared region Primary and secondary aliphatic amides give rise to several absorptions in the near-infrared region w23–25x. These absorptions have been assigned to combinations of the 2 n C5O or n NH vibrations with the amide II or amide III vibrations. In the near-infrared spectrum of DMF-d7 , only very weak absorptions are observed between 5500 and 4500 cmy1 . The bands at 4967 and 4927 cmy1 can be assigned to the second overtone of the n C5O modes observed at 1699 and 1665 cmy1 in the fundamental region. This assignment is strengthened by the fact that only one absorption at 5016 cmy1 is observed in the spectrum of DMF. Further, the most intense absorptions observed at 4449, 4407, 4238, and 4172 cmy1 are due to the overtones of the n C-D vibrations. Owing to the overlapping with the strong absorptions of the phenols, the perturbation of the DMF-d7 modes resulting from complex formation could not be observed. This is shown in Fig. 2 which reproduces the near-infrared spectrum of 4-Cl phenol
and the 4-Cl phenol–DMF-d7 complex. In free 4-Cl phenol, the n 02 ŽOH. vibration is observed at 7048 cmy1 and the absorptions observed at 5203, 5032, 4926, 4869, and 4781 cmy1 have been assigned to combinations involving the n ŽOH. vibration and internal modes Ž n 8 a, n 19b, n 14, n C-O and d OH. of 4-Cl phenol w26x. In the DMF-d7 complex, the intensity of these absorptions decreases and this confirms our previous assignment. Very recently, a very similar assignment of jet-cooled phenol has been proposed w27x. The n 02 ŽOH . . . O. vibration is observed at 6270 cmy1 and the anharmonicity of this vibration will be discussed in the next section. For all the complexes, an overall intensity increase of the absorption in the 5750–4700 cmy1 region is observed. This is clearly shown in Fig. 2 and in Fig. 3 which reproduces the spectrum between 5300 and 4700 cmy1 of phenol–DMF-d7 solutions taken at different DMF-d7 concentrations. In this complex also, the absorptions observed at 5209, 5080, 4949, 4867, and 4783 cmy1 in free phenol decrease in the DMF-d7 complex and new broad bands are observed at about
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
45
Fig. 2. Near-IR spectrum Ž8000–4000 cmy1 . of ŽA. 4-Cl phenol Ž c s 0.06 mol dmy3 .; ŽB. 4-Cl phenol Ž c s 0.06 mol dmy3 . and DMF-d7 Ž c s 0.18 mol dmy3 ., Solvents carbon tetrachoride, Path lengths 5 cm; ŽC. difference spectrum between B and A.
Fig. 3. Near-IR spectrum Ž5300–4700 cmy1 . of the complex between phenol and DMF-d7 . Concentration of phenols 0.04 mol dmy3 . Concentration of DMF-d7 : Ž1. s 0, Ž2. s 0.036 mol dmy3 , Ž3. s 0.054 mol dmy3 , Ž4. s 0.077 mol dmy3 , Ž5. s 0.123 mol dmy3 .
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
46
5440, 5275, 5145, 5010, and 4860 cmy1 . The intensity of these bands is very weak. The extinction coefficient of the band at 5010 cmy1 estimated from the absorbance and the concentration of complexes is about 0.2 moly1 cmy1 and is much smaller than the extinction coefficient of the free phenol absorption at 4949 cmy1 which is about 1.25 moly1 cmy1 . No submaxima could be observed between 6100 and 5750 cmy1 owing to overlapping with the 2 n ŽCH. vibrations of the phenols. The broad absorption, somewhat similar to the ‘continuum’ observed in the mid-infrared region w28x, but of much lower intensity, is intriguing and cannot be explained by proton transfer in the second vibrational excited state. The complexes studied in this work are characterized by ˚ and the barrier RŽO . . . O. distances of about 2.75 A height of proton transfer is in this case higher than 10,000 cmy1 w29x. In order to provide some explanations on the observed sub-maxima, the frequencies of some relevant absorptions observed in the mid-infrared region of free phenol and the phenol–DMF-d7 complex are indicated in Table 3. As shown by these data, the complex absorptions cannot be assigned to combinations involving the n ŽOH . . . O. and n 8 a, n 19b, n 14 or n C-O vibrations. In the spectrum of self-associated aliphatic alcohols, the broad absorption observed between 4600 and 5000 cmy1 has been assigned to the n ŽOH . . . O. q d ŽOH. combination w30x. In the present complex, this combination should give an absorption around 4550 cmy1 which
Table 3 Mid-infrared and near-infrared data Žcmy1 . observed in free phenol and the phenol-DMF-d7 complex Free phenol
Phenol–DMF-d7 complex
Assignment
3611 1608 1597 1499 1470 1342 1255 1178 310
3314 1606 1594 1501 1473 1345a 1267 a 1231 690 170 b
n ŽOH. n 8a n 8b n 19 a n 19b n 14 n C-O d ŽOH. g ŽOH. ns ŽH . . . O.
a b
Overlapping with a DMF-d7 absorption. From Ref. w30x.
was not observed. A simultaneous excitation w7,30– 32x involving the n ŽOH . . . O. and n ŽC5O. vibrations should result in an absorption at 4967 cmy1 and must also be ruled out. The distances between the absorptions observed between 5450 and 4860 cmy1 is between 130 and 160 cmy1 . The intermolecular stretching vibration Ž ns . which is observed at 150 cmy1 in the complex phenol-N-methylacetamide w33x must be of the same of the same order of magnitude for the present complex, the strength of the interaction and the reduced mass of the interacting molecules being about the same w34x. Combination between the high and low frequency modes 2 n ŽOH . . . O. " nns is perhaps at the origin of the broadness and the band shape. To the best of our knowledge, the combination between the first overtone of the stretching vibration and the intermolecular mode has been observed only for the dimethylether–hydrogen fluoride complex in the gas phase w35x and the Millen satellite bands seem to have a higher intensity in the overtone region than in the fundamental one w36x. In the present complexes also, some fine structure of the n ŽOH . . . O. stretching vibration is also observed in the fundamental region, between 3000 and 2500 cmy1 . Sheppard w37x has pointed out that the proton will be on the average, nearer to the base in the Õ s 1 state than in the Õ s 0 state and as a consequence, the hydrogen bond will be stronger in the first excited vibrational state than in the fundamental one. These effects are expected to be more pronounced in the second vibrational state and the coupling between the high and low frequency modes more effective. Thus, in the present complexes, the first overtone of the n ŽOH . . . O. vibration is very broad and shows a pronounced asymmetry at the low frequency side. The subbands are tentatively assigned to combinations 2 n ŽOH . . . O. y nns . Measurements at low temperature where the hot bands disappear would confirm our interpretation. As suggested by one of the referees, the band substructure could also be explained by the well-known ‘Evans Holes’ reported many times for strong hydrogen bonds. This explanation cannot be ruled out, but seems less probable because the complexes studied in this work are not very strong and in the fundamental region, the n 01 ŽOH. band does not show a sudden interruption. However, in both interpretations, the anharmonicity
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49 Table 4 Near-infrared data for free phenols and their complexes with DMF-d7 . Phenol 4-OCH 3 phenol phenol 4-Cl phenol 3-Br phenol 3,4-diCl 2 phenol 3,5-diCl 2 phenol
n 02 ŽOH. f 7057 7052 7048 7041 7040 7033
n 02 ŽOH . . . O. a
Xb
6445 6380 6270 6260 6185 6140
117 124 141 140 144 150
D n ŽOH. e 199 219 221 225 250 251
a
Experimental error on the absorption maximum"15 cmy1 . The experimental error on the anharmonicities estimated from the errors on the experimental frequencies in the mid- and near-infrared region is "12 cmy1 . Anharmonicity of the n ŽOH . . . O. vibration Ždata in cmy1 . and harmonic frequency shift.
b
is an important factor in the band-shaping mechanism. 3.2.2. Anharmonicity of the n (OH . . . O) Õibration and hydrogen bond strength In a solution of 4-Cl phenol and DMF-d7 , the intensity of the free n 02 ŽOH.absorption at 7048 cmy1 decreases and a new band assigned to the n 02 ŽOH. vibration in the complex is observed at 6270 cmy1 . The maximum of this absorption has been determined by subtracting the spectrum of phenol at the same concentration to the spectrum of the ternary solution. The error on the position of the n 02 ŽOH . . . O. absorption is "15 cmy1 . Table 4 lists the experimental n 02 ŽOH. values in free phenols and their complexes with DMF-d7 , the
Fig. 4. D n ŽOH. Žcmy1 . as a function of y DH ŽkJ moly1 . experimental frequency shifts or harmonic frequency shifts.
47
Table 5 Enthalpies of complex formation and anharmonicities of the n ŽOH . . . O. vibration for complexes between phenols and Nmethyldiacetamide Ždata from Ref. w14x. Phenol
y DH ŽkJ moly1 .
X Žcmy1 .
3,4-diCH 3 phenol 4-CH 3 phenol phenol 4-Cl phenol 4-Br phenol 3-Br phenol 3,4-diCl 2 phenol 3,5-diCl 2 phenol
19.3 20.6 21.4 21.6 23.4 23.5 23.7 24
80 83 80 85 85 95 102 106
anharmonicity constants along with the harmonic frequency shifts Ž D n e ŽOH... The harmonic shifts are computed from Eq. Ž5. taking a value of 85 cmy1 for the anharmonicity of the n ŽOH. vibration of the free phenols w26x. Inspection of the results of Table 2 shows that the anharmonicities increase with the hydrogen bond strength. The correlation between the enthalpies of complex formation and the harmonic frequency shifts can be written as f
yD H s a q b Ž n 01 Ž OH . y n 01 Ž OH . . . O . q2 X c y 2 X f ,
exp
Ž 7.
where X f and X c refer to the anharmonicities of the n ŽOH. vibration in the free and complexed molecules.
Fig. 5. X Žcmy1 . as a function of y DH ŽkJ moly1 . for complexes involving phenols and DMF-d7 Žthis work. and N-methylsuccinimide ŽRef. w14x..
48
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49
In the present case, the validity of the Badger– Bauer correlation can be explained by the fact that, as indicated by the data of Tables 1 and 4, the increase of anharmonicity resulting from hydrogen bonding is roughly proportional to the enthalpy of complex formation. As suggested by Sandorfy w1x, the hydrogen bond frequency shift reflects not only the change in the equilibrium distance that occurs when the hydrogen bond is formed, but also the accompanying changes in the values of the anharmonicity constants. The validity of the Badger–Bauer correlation thus implies that the change in anharmonicity is roughly proportional to the change in equilibrium distance. In the present case, the Badger–Bauer correlation can be written as e
yD H s 10.50 q 0.063D n Ž OH . Ž r s 0.978 . .
Ž 8. The correlation between the enthalpies of complex formation and the experimental or harmonic frequency shifts is illustrated in Fig. 4. Table 5 lists the values of the anharmonicities and the enthalpies of complex formation obtained recently for the complexes between N-methyldiacetamide CH 3 CŽ5O.NCH 3 CŽ5.OCH 3 and phenol derivatives w14x. These complexes belong to the same family and are somewhat weaker than the DMF complexes. Fig. 5 shows that the increase of the anharmonicity with the hydrogen bond strength can be extended in a broader domain. The scatter of the points observed in Fig. 5 is due to the experimental errors on the anharmonicity Ž"12 cmy1 . and on the enthalpies on complex formation Ž"1 kJ moly1 .. Also, as previously discussed, the second-order perturbation theory is perhaps not entirely valid for these hydrogen bonds of medium strength. For the weakest hydrogen bonds, the anharmonicity is of the same order of magnitude as that of the free n ŽOH. vibration. This is agreement with the experimental data of Sandorfy on weak hydrogen bonds existing as for instance in self-associated aliphatic amines. For these weak hydrogen bonds, the anharmonicity of the n ŽNH. vibration is nearly the same in the monomers and self-associated species w12x. Our results are in qualitative agreement with theoretical data on OH . . . O systems w3x. The anharmonicity of
the n ŽOH . . . O. vibration has been computed as a function of the RŽO . . . O. distance. For weak and medium hydrogen bonds, the anharmonicity increases slightly with decreasing RŽO . . . O. distances. Acknowledgements The authors thank the Fund of Scientific Research-Vlaanderen for financial support. This work has been done in the frame of the bilateral cooperation between Poland and Flanders. Z.P. thanks the Flemish Community for a post-doctoral fellowship. References w1x C. Sandorfy, The hydrogen bond, Recent Developments in Theory and Experiments: II. Structure and Spectroscopy, in: P. Schuster, G. Zundel, C. Sandorfy ŽEds.., North-Holland, Amsterdam, 1976, p. 645. w2x W.A.P. Luck, T. Wess, J. Mol. Struct. 270 Ž1992. 229. w3x W.A.P. Luck, T. Wess, Can. J. Chem. 69 Ž1991. 1919. w4x C. Sandorfy, Bull. Acad. Pol. Sci. 43 Ž1995. 7. w5x C. Sandorfy, Top. Curr. Chem. 120 Ž1984. 42. w6x W.A.P. Luck, Intermolecular forces, An Introduction to Modern Methods and Results, in: P. Huyskens, W.A.P. Luck, Th. Zeegers-Huyskens ŽEds.., Springer-Verlag, Berlin, 1991, p. 157. w7x M. Asselin, C. Sandorfy, J. Chem. Phys. 52 Ž1970. 6130. w8x M. Asselin, C. Sandorfy, Chem. Phys. Lett. 8 Ž1971. 601. w9x C. Bourderon, J.J. Peron, C. Sandorfy, J. Phys. Chem. 76 Ž1972. 864. w10x M.C. Bernard-Houplain, C. Sandorfy, Can. J. Chem. 51 Ž1973. 1075. w11x M.C. Bernard-Houplain, C. Sandorfy, Can. J. Chem. 52 Ž1973. 3640. w12x M.C. Bernard-Houplain, C. Sandorfy, J. Chem. Phys. 56 Ž1972. 3412. w13x M. Rospenk, Th. Zeegers-Huyskens, J. Phys. Chem. 101 Ž1997. 8428. w14x N. Leroux, C. Samyn, Th. Zeegers-Huyskens, J. Mol. Struct., in press. w15x M.D. Joesten, R.S. Drago, J. Am. Chem. Soc. 84 Ž1962. 3817. w16x H. Fritsche, Ber. Bunsenges. Physik. Chem. 68 Ž1964. 459. w17x G. Durgaprasad, D.N. Santhyanarayana, C.C. Patel, Bull. Chem. Soc. Japan 44 Ž1971. 316. w18x E.W. Randall, C.M. Yoder, J.J. Zuckerman, Inorg. Chem. 5 Ž1966. 2240. w19x D. Steele, A. Quatermain, Spectrochim. Acta 43A Ž1987. 781. w20x X. Zhou, J.A. Krauser, D.R. Tate, A.S. Van Buren, J.A. Clark, P.R. Moody, R. Liu, J. Phys. Chem. 100 Ž1996. 16822.
Z. Pawelka, Th. Zeegers-Huyskensr Vibrational Spectroscopy 18 (1998) 41–49 w21x C. Dorval, Th. Zeegers-Huyskens, Spectrochim. Acta 29A Ž1973. 1805. w22x A. Foldes, C. Sandorfy, J. Mol. Spectrosc. 20 Ž1966. 262. w23x S.E. Krikorian, M. Marpour, Spectrochim. Acta 29A Ž1973. 1233. w24x S.E. Krikorian, Spectrochim. Acta 37A Ž1981. 745. w25x P.J. McNeilly, T.A. Andrea, S.E. Krikorian, Spectrochim. Acta 48A Ž1992. 1437. w26x M. Rospenk, N. Leroux, Th. Zeegers-Huyskens, J. Mol. Spectry. 183 Ž1997. 245. w27x S.-I. Ishiuchi, H. Shitomi, K. Takazawa, M. Fujii, Chem. Phys. Lett. 283 Ž1998. 243. w28x G. Zundel, Ref. w1x, p. 687.
w29x w30x w31x w32x w33x w34x w35x w36x w37x
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T. Saitoh, K. Mori, R. Itoh, Chem. Phys. 60 Ž1981. 161. M. Asselin, C. Sandorfy, Can. J. Chem. 49 Ž1971. 1539. A. Burneau, J. Corset, J. Chem. Phys. 56 Ž1972. 662. A. Burneau, Spectrochim. Acta 45A Ž1989. 1305. C. Dorval, Th. Zeegers-Huyskens, Spectrochim. Acta 30A Ž1974. 1757. E. Knozinger, O. Schrems, in: J.R. Durig ŽEd.., Vibrational ¨ Spectra and Structure, Vol. 16, Elsevier, 1987. J.W. Bevan, B. Martineau, C. Sandorfy, Can. J. Chem. 57 Ž1979. 1341. J. Arnold, D.J. Millen, J. Chem. Soc., 1965, 503. N. Sheppard, in: D. Hadzi ŽEd.., Hydrogen Bonding, Pergamon, London, 1959, p. 85.