J. Phys. Chem. 1991, 95, 2770-2780
2770
Matrix Isolation Studies and ab Initio Calculations of the Vibrational Spectra of Complexes of Water with 3-Hydroxypyridine W. B. Person,**+Janet E. Del Bene,**$W. Szajda: Krystyna Szczepaniak: and M. Szczesniakt Department of Chemistry, University of Florida, Gainesville, Florida 3261 1 - 2046, and Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555 (Received: September 25, 1990; In Final Form: January 4, 1991)
Experimental infrared spectra of a mixture of 3-hydroxypyridineand water in an argon matrix formed by condensation of a gaseous mixture onto a cold window at 15 K exhibit new bands from the two types of 1:l complexes formed between the two molecules. The first of these is H20--H0(3HP), in which the water molecule acts as a proton acceptor from the OH group of the 3-hydroxypyridine, while the second is HOH-N(3HP), with the water molecule acting as a proton donor to the N atom of the 3-hydroxypyridine. The vibrational spectra of these complexes are recorded and compared with the spectra of the isolated H 2 0 and 3-hydroxypyridinemolecules to observe the changes that result from complex formation. The relative concentrations of each complex are estimated from the observed relative intensities and the calculated values of the absolute intensities of the corresponding bands. In order to verify these experimental observations and to aid in their interpretation, ab initio calculations at the HF/6-31G(d) level have been carried out to predict the infrared spectra of the monomers and of the complexes. In general, there is good agreement between the experimental and calculated spectra. The calculations provide a basis for the assignment of the experimental spectra of the monomer and for both complexes. According to the calculations the intramolecular vibrations of the 3-hydroxypyridine do not change appreciably on complex formation, except for those bonds directly involved in the hydrogen bonding. Analysis of the calculated changes in terms of changes in the intensity parameters and in the force constants for the OH stretching mode (for the proton donor group in the hydrogen bond) shows striking similarities for the two complexes studied here and the water dimer.
SCHEME I
Introduction
It is certainly true that the chemistry of life occurs in a water medium. In particular for nucleic acid bases occurring in nucleotides and polynucleotides, the water medium is believed to be responsible not only for the primary tautomeric structures of the monomers, but also for the stabilization of the secondary and tertiary structures of the polymers (ref 1, and references therein). As a result, there have been a number of studies of the interactions between H 2 0 and proteins or nucleic acid bases [e.g., ref 2, and references therein]. We are conducting experimental studies of the infrared spectra of such one-to-one complexes between H 2 0 and nucleic acid bases trapped under somewhat controlled conditions in an argon matrix. It is expected that such studies will provide vibrational information about isolated complexes in a hydrophobic environment which can be directly compared with quantum mechanical calculations. Such comparisons aid in the interpretation of the experimental data and serve as a necessary verification of the reliability of the calculations. In cases of the nucleic acid bases where there are several different possible sites where water molecules might interact, such experimental studies can provide information about which of these possible complexes do form, and about the relative concentrations of the different complexes. The spectral properties of the complexes are expected to be correlated with their interaction energies. The 3-hydroxypyridine (3HP) molecule may serve as a model for the "rare" hydroxy tautomeric forms of pyrimidine and purine bases, such as cytosine, guanine, 9-methylguanine, isocytosine, and others, so that we chose to study its complexes with water first as models for the more complicated complexes involving the nucleic acid bases. The oxo form of this pyridine derivative may not be described in terms of simple nonionic Kekule structures, and the energy of the zwitterion oxo form has been calculated to be more than 20 kcal mol-' higher than that of the hydroxy form.3 No trace of any absorption band for the oxo tautomer has been observed in the study of the infrared spectra of matrix-isolated 3HP which we report here and in ref 4. In the pure crystal, X-ray diffraction studies find that the molecules are in the hydroxy form,5 but it has been reported6 that the zwitterion 'University of Florida. Youngstown State University.
*
0022-3654/9 1/2095-2770$02.50/0
&
oxo form is predominant in water solutions. Three different types of complexes of 3HP with a single H 2 0 molecule (Scheme I) are possible: one (I) in which the H 2 0 molecule acts as a proton donor toward the N atom in the pyridine ring; another (11) in which the O H group from 3HP acts as a proton donor to the oxygen in the H 2 0 molecule; and one (111) in which the H 2 0 molecule acts as a proton donor toward the oxygen atom of the O H group of the 3HP. Formation of these complexes is expected to change mainly the frequencies and intensities of the O H stretch of the 3HP in I1 and of the H 2 0 molecule in the other two complexes. We would expect the rest of the 3HP spectrum to be relatipely less affected, unless the one-to-one complex formation resulted in tautomerization to the oxo form, in which case the spectrum would be radically changed for the complex relative to the monomer. Studies of complexes between H 2 0 and different molecules formed in argon matrices (for example, refs 7 and 8), as well as (1) (a) Saenger, W . Annu. Rev. Biophys. Chem. 1987,16,93. (b) Westhof, E. Annu. Rev. Biophys. Chem. 1988, 17, 125. (2) (a) Dickerson, R. E.; Drew, H. R.; Conner, B. In Biomoleculur Stereodynamics; Sarma, R. H., Ed.; Adenine Press: New York, 1988; pp 1-34. (b) Kennard, 0. J. Biomol. Struct. Dyn. 1985, 3, 205. (c) Finney, J. L.; Goodfellow, J. M.; Howell, P. L.; Vovell, F. J. Biomol. Struct. Dyn. 1985, 3,
599. (3) Scanlan, M. J.; Hillier, I. 1983,105, 3568.
H.; McDowell, A. A. J . Am. Chem. Soc.
(4) Szajda, W.;Szczepaniak, K.; Leszczynski, J. L.; Person, W.B., to be published. ( 5 ) Ohms, U.; Guth, H.; Treutman, W . Z . Kristallogr. 1983, 162, 299. (6) Katritzky, A. R. Handbook of Heterocyclic Chemistry; Pergamon: New York, 1985; pp 47-50.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2771
Complexes of Water with 3-Hydroxypyridine calculations of the vibration of spectra have been made before and have been successful enough that we anticipated that it would be possible to examine the water complexes of 3HP.
Methods Experimental Details. The matrix was prepared by slow deposition of a gaseous mixture of argon with a small amount of water and of 3HP onto a cold window held in an evacuated Displex (CSA 202E) closed-cycle H e cryostat. The window is held at about 15 K throughout the deposition and spectroscopic study. The gaseous mixture is formed by flowing argon over a glass furnace containing solid 3 H P heated to about 80 OC, causing it to sublime into the argon to form a mixture of 3HPargon of about 1:lOOO. This matrix ratio can be adjusted by controlling the rate of sublimation and the flow rate of the argon. The ratio is judged by the appearance of the spectrum in light of experience with numerous samples. The technique has been described elsewhere? The pressure in the system before and after deposition is measured to be less than 10” Torr, and the ability to control the concentration of water in the argon can be judged by reference to the study of the spectrum of water alone in argon reported elsewhere,I0 where the spectrum of a matrix containing water at a matrix ratio of less than 1:50000 is shown. The ability to carry out this study of one-to-one complexes of water with 3 H P is due primarily to the very high sensitivity of the Nicolet Model 740 FTIR spectrometer used. It was possible to study spectra with a maximum absorbance of less than 0.001 at a signal-tcmoise ratio still better than 10 to 1. Hence we were able to study quite readily thin films of very dilute matrices. Our studies were made under more or lass routine operating conditions using the standard DTGS detector at a resolution of 1 cm-I, usually with 64 up to 500 scans. Water vapor was removed from the spectrometer by flushing with dry N2,which reduced it to the point where the absorbance of individual lines in the spectra was small enough that it could be removed by subtraction to obtain the spectra shown here in the figures. The sample of 3 H P was obtained from the Aldrich Chemical Co. It was purified by sublimation. A sample of deuterated 3HP(OD) with the O ( H ) replaced by O D was obtained by recrystallization of the 3 H P from D 2 0 . In order to be able to identify with certainty the absorption bands due to water, we carried out a thorough study of matrices containing so little water that only the monomer was present, with other matrices containing higher and higher concentrations of water so that the spectra of the dimer and of higher polymers of water could be clearly identified. These results have been reported elsewhere.I0 Also we have carried out studies of the infrared spectra of matrix isolated 2- and 4-hydroxypyridines which have been compared with the spectra of the isolated 3 H P to obtain a complete assignment of its spectrums4 Calcularions. Gradient optimization techniques” were employed to fully optimize the geometries of H 2 0 , 3HP, and the two complexes of H 2 0 with 3 H P at the single-determinant HartreeFock level using the split-valence 6-3lG(d)l2J3 basis set with polarization functions on non-hydrogen atoms. Total energies for each of the optimized equilibrium structures (no imaginary frequencies) were then computed with correlation using second-order ~~
~
~
(7) (a) Engdahl. A.; Nelander, 9 . J . Chem. Phys. 1989, 91, 6604. (b) Maes, G. Bull. Soc. Chim. Belg. 1981, 90,1093. (c) Barnes, A. J. J . Mol. Struct. 1983,100,259. (d) Schriver, L.; Burneau, A.; Perchard, J. P. J. Chim. Phys. 1985,82,9. (e) Suzer, S.;Andrews, L. J. Chem. Phys. 1988,88,916. (8) (a) Latajka, Z.; Scheiner, S. J . Phys. Chem. 1990, 94, 217. (b) Scheiner, S. In Theoretical Models of Chemical Bonding; Maksic, Z. B., Ed.; Springer-Verlag: Berlin, in press. (9) (a) Szczesniak, M.; Nowak, M. J.; Rostkowska, H.; Szczepaniak, K.; Person, W. 9.; Shugar, D. J. Am. Chem. Soc. 1983, 105, 5969. (b) Szczesniak, M.; Szczepaniak, K.; Kwiatkowski, J. S.; KuBulat, K.; Person, W. 9 . J . Am. Chem.Soc. 1988. 110, 8319. (IO) Szczesniak, M.: Szajda, W.; Szczepaniak, K.; Person, W. B., to be published. (1 1) (a) Schlegel, H. 9. Ph.D. Thesis, Queen’s University, 1975. (b) Pulay, P. Mol. Phys. 1969, 17, 197. (12) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acra 1973, 28, 213. (13) DlI1, J. D.;Pople, J. A. J . Chem. Phys. 1975, 62, 2921.
many-body Moller-Plesset perturbation theory (MP2)Ie1’ with the 6-31+G(d,p)18J9basis set, which has polarization functions on all atoms and diffuse s and p functions on non-hydrogen atoms. Analytical second derivatives of the electronic energy with respect to the nuclear coordinates were calculated at HF/6-3 lG(d) for each optimized structure. The force constants obtained were used in the GAUSSIAN 88 programZoto determine harmonic vibrational frequencies and associated zero-point energies. The frequencies, intensities, and potential energy distributions ( P E D S ) ~were ’ recalculated from the Cartesian force constants and the atomic polar tensors from GAUSSIAN 88, using programs written by KuBulat22 and described el~ewhere.2~ The hydrogen bond enthalpy at 15 K was computed as described previouslyz4 for the reaction
H20 + 3 H P
4
3HP...H20
using MP2/6-3 1+G(d,p) electronic energies and HF/6-3 1G(d) zero-point energies. At 15 K, the thermal energy terms contribute only +0.1 kcal/mol to the hydrogen bond enthalpy. All calculations were performed on the Cray Y-MP8/864 computer at the Ohio Supercomputer Center and the University of Florida Chemistry Department MicroVax. Comparison of Theoretical and Experimental Spectra. It is well-known that the calculated Hartree-Fock 6-31G(d) frequencies of normal modes are about 10% higher than those observed experimentally. This error is due in part to the use of the Hartree-Fock approximation and in part to the fact that the computed frequencies are harmonic frequencies. Therefore, in order to give a more straightforward comparison between the calculated and experimental frequencies, the calculated frequencies have been multiplied by a single “scaling factor” of 0.89. Although this scaling factor brings most of the calculated frequencies into good agreement with experimental values, there is still a large discrepancy for the calculated OH and CH stretching vibrations of 3HP compared with experiment. We believe that this remaining discrepancy, particularly for the hydrogen-bonded OH stretch, is due primarily to the larger anharmonicities of these modes. (Compare the values for the anharmonicity constants given by HerzbergZ5’ for the diatomic C H , O H , C N , and C O molecules and the review of the anharmonicity of hydrogen-bonded OH by Sand~rfy.~~~) Experimental intensities have been measured in this study only as relative values, because of the inherent difficulty in measuring the precise concentration and path length for the matrix samples. To compare these relative experimental intensities with the corresponding calculated values, we have chosen the integrated absorbance of the O H stretching mode to be equal to the absolute intensity calculated for this mode. We then multiplied the experimental integrated absorbances measured for all the other (14) Pople, J. A.; Binkley, J. S.; Seeger, R. Inr. J . Quantum Chem., Quantum Chem. Symp. 1976, I O , 1 . (15) Krishnan, R.; Pople, J. A. Inr. J. Quantum Chem. 1978, 14, 91. (16) Bartlett, R. J.; Silver, D. M. J . Chem. Phys. 1975, 62, 3258; 1976, 64, 1260. (17) Bartlett, R. J.; Purvis, G. D. Inr. J . Quantum Chem. 1978, 14, 561. (18) Spitznagel, G. W.; Clark, T.; Chandrasekhar, J.; Schleyer, P. v. R. J . Comput. Chem. 1982,3, 363. (19) Clark, T.;Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J . Comput. Chem. 1983, 4 , 294. (20) Frisch, M. J.; Head-Gordon, M.; Schlegel, H. 9.; Raghavachari,K.; Binkley, J. S.; Gonzalez, C.; DeFrees, D. F.; Fox, D. J.; Whitehead, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R.; Kahn, L. R.; Stewart, J. J. P.: Ruder, E. M.; Topiol, S.;Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1988. (21) (a) Morino, Y.; Kuchitsu, K. J . Chem. Phys. 1952, 20, 1809. (b) Keresztury, G.; Jalsovszky, G. J . Mol. Srrucr. 1973, 10. 304. (22) KuBulat, K. Ph.D. Dissertation, University of Florida, 1989. (23) Person, W. 9.; KuBulat, K. J . Mol. Strucr., to be published. (24) Del Bene, J. E. In Molecular Structures and Energetics; Liebman, J. F., Greenberg, A,, Eds.;VCH Publishers, Inc.: Deerfield Beach, FL, 1986; VOl. I. (25) (a) Herzberg, G. Molecular Spectra and Molecular Srructure I . Diatomic Molecules, 2nd ed.;Van Nostrand: New York, 1950. (b) Sandorfy. C. In The Hydrogen Bond. Recent Developments in Theory and Experin”; Schuster, P., Zundel, G., Sandorfy, C., Eds.;North Holland Publishing Co.: Amsterdam, 1976; Vol. 2, p 613.
2772 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 normal modes by the ratio of the calculated intensity for this OH stretch to the experimental integrated absorbance of the OH stretch. In this way we obtain the experimental values for the absolute intensities for all the observed modes of the molecules studied here and reported in the tables. The experimental measurement of the intensities has considerable uncertainty, since it does involve some deconvolution of overlapping bands. Given both the experimental and theoretical limitations, we believe that the agreement between the intensities measured as described here with the calculated intensities is as good as could be expected. In order to interpret the PEDs, it is necessary to know the definition of the symmetry coordinates; these follow the suggestion by PulayZ6for cyclic molecules, and they are given for 3 H P in Table I, which also shows the definition of the internal coordinates. The use of PEDS in visualization of normal coordinates is helpful in discussing molecules such as 3 H P which are related chemically to other molecules to which they will be compared. It is generally easier to visualize a vibration for such a molecule in terms of combinations of symmetry coordinates (PEDs) than it is to visualize the vibration in terms of squared Cartesian displacement coordinates, such as those given in the output for GAUSSIAN88, especially for large molecules such as 3 H P and its complexes. However, we find that the resultant redundancy problems sometimes reduce the physical significance of the ring vibrations, for example, and the standard definitions of out-of-plane coordinates are sometimes quite awkward, especially (in our case) for the vibrations of the complexes. For this reason, the symmetry coordinates given in Table I have somewhat nonconventional definitions. The definitions of symmetry coordinates of the H 2 0 molecule (symmetric and asymmetric OH stretches and the bending mode) are standard for the monomer and for the H20-H0(3HP) complex but need some discussion for the complex [H20N(3HP)I in which the water acts as a proton donor. In this complex, the two OH groups are different (one is “free” and the other is “bonded”) so that the definition of symmetry coordinates in Table I is different for water in the two complexes. Finally, we note that the low-frequency “intermolecular” modes (which we designate as “modes involving both H 2 0 and 3HP” in our tables) are the vibrations which arise from the three translational plus the three rotational degrees of freedom of the water molecule when the complex forms. In these modes, the water molecule moves with respect to a more or less stationary (except to prevent overall rotation or translation of the complex) 3 H P molecule. PEDS expressed in terms of standard internal coordinates are not helpful in visualizing these motions. Our definitions of these symmetry coordinates in Table I is an attempt to describe in words these motions as given by the Cartesian displacement coordinates. The PEDS for these low-frequencymodes in the other tables are then defined to be 100% pure motion of the defined “symmetry coordinates”. Results and Discussion
Isolated Monomers. We shall begin our presentation of the experimental results with the infrared spectra of the noninteracting molecules (monomers). Figure 1 shows the experimental spectrum (bottom) compared with the calculated spectrum (top, vertical lines) for the H20molecule. In the experimental spectrum of the H 2 0 molecule isolated in an argon matrix at 15 K we see several vibration-rotation lines (frequencies and intensities are listed in Table 11) for each mode, while only single lines are predicted by the calculation for the pure vibrational transitions of the nonrotating H 2 0 monomer. The scaled calculated frequencies are compared with (and found to be very close to) the band centers of the experimental vibration-rotation bands (see Tables I1 and 111). As seen in Table 111, the experimental intensities agree quite well with the intensities calculated at HF/6-31G(d). The agreement with the experimental intensities for the water vibrations
Person et al. H20 CALC
m
Figure 1. Comparison of the experimental IR spectrum of H 2 0 monomer isolated in an argon matrix at about 15 K with the IR spectrum of the H 2 0 monomer predicted by an ab initio HF/6-31G(d) calculation. The experimental spectrum corresponds to the absorption by the rotating H 2 0 monomer; hence, several vibration-rotation lines are observed for each vibrational transition predicted for the rotating monomer.
is, of course, even better for calculations at higher levels of theory.27 Figure 2, a and b, presents the experimental spectra of 3 H P and 3HP(OD) monomers, compared with the calculated spectra for these isolated molecules. As may be seen in Figure 2, the spectral patterns calculated for these molecules are very similar to those observed in the experimental spectra, although the bands in many regions of the experimental spectra are observed to be split into several components, due to Fermi resonance and to matrix site effects not included in the calculations. Table I1 presents the details of the frequencies, intensities, and the assignment of all bands observed in the experimental infrared spectrum of H 2 0 and of 3 H P monomers isolated in an argon matrix. Table 111 presents in its first four columns the calculated frequencies, intensities, and potential energy distributions (PEDs) for H 2 0 and 3 H P monomers. Below each calculated frequency and intensity in this table are the experimental frequencies (of the main bands in a multicomponent band) and the total intensity measured experimentally for each normal mode. Table IV collects the calculated and experimental values of the frequencies, intensities, and PEDS for 3HP(OD) monomers. The experimentally observed frequencies and intensities are given in parentheses below the corresponding calculated values. The frequencies and intensities of the most important components of overlapped (or multicomponent) bands are underlined. The distribution of intensities predicted in the calculated spectrum differs from the experimental pattern more in the region near 1200-1500 cm-’ than is the case for other regions of the spectrum. We believe that this is due to the strong normal-mode coupling in this region and to the level of theory used to calculate the force constants. In Figure 2 the intensities of the C H stretching vibrations (near 3000 cm-I) seem to be overestimated in the calculation, but when all the components of the observed multicomponent band are added to get the total experimental intensities of each normal mode in this region, the experimental and theoretical results are in much better agreement (see Table 111). Due to technical limitations, we were not able to study the experimental spectra below 500 em-’. Because of the good agreement observed between the calculated and experimental spectra in the higher frequency range (4000-500 cm-I), we expect
(26) (a) h l a y , P.; Fogarasi, G.;Pang, F.; Boggs, J. E . J . Am. Chem. Soc.
1979, 101. 2550. (b) London, 1976.
See also: Califano. S. Vibrational States; Wiley:
(27) Stanton, J. F.;Lipscomb, W. N.; Magers, D. H.; Bartlett, R. J. J. Chem. Phys. 1989, 90, 3241,
Complexes
of Water with 3-Hydroxypyridine
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2773
TABLE I: Symmetry Coordinates for H20, for 3-Hydroxypyridine (3HP), and for Their Two Complexes (I and 11) Studied0 H
symmetry coordinateb For H 2 0 Monomer or H2O*-H0(3HP) A ~ O H+I b O H 2 A ~ O H-I &OH2
SI’
si
So’ =
description‘ OH st a OH st HOH be (same for all)
s
WHOH
For HOH-N(3HP)
f OH st bo OH st
NlC2 st C2C3 st c 3 c 4 st c 4 c s st CSC6 st NlC6 st C 3 0 st OH st (for OH in 3HP) C2H st C4H st CSH st C6H st Ri de 1 Ri de 2 Ri de 3 HOC be C 3 0 be C4H be CSH be C6H be C2H be o Ri de 1 o Ri de 2 o Ri de 3 C30 wa C2Hwa C4H wa CSH wa C6H wa HO tor Coordinates Involving Both Molecules For the HOH-.N(3HP) Complex
SI” = A812
s,ll
AYOHN
I
S3”= Ar13 S4”= ATNO SJl’ = in-plane ‘butterfly”
561)= out-of-plane ‘butterfly”
OH-N be ( R J d OH-N O-*N st o wa (R,)d HO tor (H20) about N-HO bond ipl butterfly be Bending of the Water Molecule against the 3HP Molecule opl butterfly wa
Wagging of the Water Molecule against the 3HP Molecule (Pivots on H in N.-HO Bond) For the H20-(3HP) Complex SI”= twist of the H 2 0 against the OH-0 bond H,O twist S,ll = H 2 0 wag parallel to the OH-0 bond H 2 0 ipl wag ( R J d 0.-0st Sj” = O.-O stretch of the H bond H 2 0 opl trans S,” = out-of-plane translation of the H20molecule against the 3HP ipl butterfly be ST = in-plane ’butterfly” rotation of the H 2 0 molecule against the 3HP S61)= out-of-plane ‘butterfly” rotation of the H 2 0 molecule against the 3HP opl butterfly wa “The in-plane internal coordinates are defined in the figure for 3HP, and the ‘in plane” and ‘out of plane (0)” refers to this isolated molecule since the complexes are not planar. The out-of-plane wagging coordinates y and torsions r,, follow the definitions given by Califano, ref 26b. Unnormalized linear combinations of internal coordinated of the appropriate symmetry, defined to be orthogonal to the redundancy conditions (e ref 27). ‘See Tables I1 and I11 for abbreviations. dThis motion is related to a rotation Rk about the x, y , or z axis of the H 2 0 monomer.
2114 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Person et al.
TABLE II: Experimental Infrared Frequencies, Intensities, and Vibrational Assignments of tbe Normal Modes of Monomers of 3HP and of H20,Isolated in an Argon MaMx at IS K
freq, cm-'
int,b km/mol assignmentc H 2 0 Monomers
3785 3776 3756 3724 371 I 3733 3669 3653 3638 I660 1636 I623 I607 1573 I590
i} 58
14
;] 3
0
I+,
1
1599 1591 -
1437 1430 1428)
%} 1331
Q3, a st,
+
+
lo
vib-rot transitions
00
-
2+,
3HP Monomers Q1, OH st
89
3020 3013 3008
I578 1488
20
I+, + 1-1
0
8 9
I5 73}1M 33
4 7}11
1277'
I249 1212 I195 I I65 1112 I100 1040
1022 101 1 969 924 887 830 820 804 80 I 704 620 542
I+, I-,
-
5 "
band center 43. a st l o -. 2-2 Q1, s st, vib-rot 1-, I+, transitions band center QI
-0
3626 3624
I+,
3HP CALC
MS MS/FR 1591 + 1488 4 2 , CH st Q3, CH st 4 4 , CH st FR 1591 + 1442 FR 1578 + 1440 FR 1578 + 1430 Q5, CH st FR 1578 + 1428 MS/FR 1346 + 267. 4 6 ; Ri st 4 7 , Ri st Q8, CH be, Ri st MS/FR 1195 + 267. Q9, Ri st, CH be MS/FR MS/FR I165 + 267' MS/FR 881 + 542 QlO, CH be MS/FR 804 + 542 MS/FR 801 + 542 MS/FR 924 + 383'? Q l l , CO st MS/FR 969 + 267' 412, HOC be, Ri st 413, CH be, Ri st MS/FR 414, Ri st, CH be FR 801 + 267* Q15, Ri st
WVEMYIL)ER 3 n ~ CRLC
9 1
3HP/AR
I
3HPlOO1 CRLC
Ql6, Ri st Q18, Ri
2 6 }32 27 4 7
de
Q l 9 , CH wa 420, CH wa 422, CO st, Ri st, Ri de MS/FR 2 X 417 MS/FR 542 + 267' 421, CH wa 423, Ri de 424, Ri de 425, Ri de
'The frequency of the strongest line in a group of overlapping bands is underlined. This frquency should be compared with the calculated frequencies in Table 111. bRelative integrated intensities, given in such a way that the experimental integrated absorbance of the OH stretch is equal to the calculated intensity of the OH stretching mode for the 3 H P monomer (see Table 111). In the case of multicomponent bands associated with one normal mode the sum of the intensities of all components should k compared with the calculated intensities given in Table 111. 'Abbreviations: vib-rot = vibration-rotation description as in ref 31; st = stretching; a = asymmetric; s = symmetric; rot = rotation; vib = vibration; be = bending; wa = wagging; de = deformation; Ri = ring; Qi = the ith normal mode (Table 111) for the monomer; FR = Fermi resonance splitting; MS = matrix site effect splitting. Here we suggest possible combinations and overtones which are in Fermi resonance with the fundamental mode. The designates calculated frequencies from Table 111; otherwise the combinations are of observed bands.
Figure 2. Comparison of the experimental IR spectra of 3HP and
3HP(OD) monomers isolated in an argon matrix at about 15 K with corresponding IR spectra for each molecule predicted by the HF/63 IG(d) calculation: the spectral region corresponding to the OH and CH stretching modes (3800-2900 cm-l) and to the OD stretching modes (2800-2600 cm-I); the spectral region (1700-100 cm-I) containing all other fundamental modes for these molecules.
the calculated spectra below 500 cm-' predict equally well the lower frequency part of the spectra. Comparison between the spectra of 3HP and 3HP(OD) allows us to verify the interpretation of the bands related to the OH and OD groups; e.g., the O H stretching at 3626 cm-I, the OD stretching at 2677 cm-I, the HOC bending at 1196 cm-I, and the DOC bending at 919 cm-I. As can be seen in Tables 111 and IV, the agreement between experimental and calculated spectra at the HF/6-31G(d) level
Complexes of Water with 3-Hydroxypyridine
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2775
F
s,
I
R
Figure 3. Comparison of the low temperature (about 15 K) IR spectra in the OH (OD) stretching region of argon matrices containing noninteracting monomers of (A) H 2 0 and (B) 3HP with the corresponding matrix spectra of mixtures of H20and 3HP at two different relative concentrations (C) and (D), and with the corresponding matrix IR spectrum (E) of a mixture of H20and 3HP(OD). The bands due to the complexes are marked by arrows. The assignment of the absorption bands of H20,3HP, 3HP(OD), and their complexes is discussed in the text and summarized in the tables. The absorption bands marked WD (at 3573 cm-I), WT (at 3516 cm-I), and WP (at 3372 cm-I) are due to H,O dimer, H 2 0 trimer, and H20polymer, respectively.
is certainly satisfactory, being much better than in the case of calculations a t the HF/3-21G level. This improvement is particularly striking for the frequencies and intensities predicted for the out of plane modes which are usually predicted much too large both for 3-21G [for example, see refs 9b and 281 and for 321G(d)29basis sets. Hydrogen-Bonded Complexes. Because the changes in the spectrum of the two molecules that form the complex are most readily observed in the O H and O D stretching regions of the spectrum, we report first in Figure 3 the experimental spectrum for 3HP and its water complexes only in the OH region from 3800 to 3350 cm-' and in the O D region from 2400 to 2500 cm-I. These regions should exhibit the characteristic changes expected for the three possible one-to-one complexes of water with 3 H P shown in Scheme 1. The changes in the rest of the spectrum of the 3 H P molecule are very small indeed, ruling out any possibility that the zwitterion oxo form is present in the argon matrix, even as a possible water complex. Some minor changes d o occur in the (28) Jaworski, A.; Szczesniak, M.; Szczepaniak, K.; KuBulat, K.; Person, W.8. J . Mol. Struct. 1990, 223, 63. (29) Rostkowska, H.;Szczepaniak. K.; Nowak, M.J.; Leszczynski, J.; KuBulat, K.; Person, W.8. J . Am. Chem. SOC.1990, 112, 2147.
spectrum of the complex in the H O H and in the COH bending regions of the spectrum in Figure 4, indicated also in Tables I1 and 111. Figure 3, C and D, shows the spectra of samples of 3 H P in an argon matrix containing differing amounts of H20. Comparing these spectra with those of the two noninteracting molecules (Figure 3, A and B) shows that there are four new bands that are attributable to the spectra of complexes of H 2 0 with 3HP. These new bands appear as a doublet at (3724,3721) cm-I, a band at 3699 cm-I, a doublet at (3433,3425), 3414 cm-', and a band at 3404 cm-I and are marked by arrows. Examination of the spectrum of H 2 0 3HP(OD) in Figure 3E shows that the bands at 3626 cm-' (free O H stretch of 3HP) and at 3433 cm-I are replaced by new bands at 2677 cm-l (free O D stretch of 3HP(OD)) and at 2537 cm-I, while the band at 3403 cm-I remains unchanged. The comparison of the spectra in Figure 3A-E allows us to make the following assignments. 1. The absorption at (3433, 3425) cm-' is related to the hydrogen-bonded OH stretching mode of the hydroxyl group of HO(3HP) in the H20-H0(3HP) complex and that at 2537 cm-' to the H20-D0(3HP) complex. 2. The band at 3404 cm-l is assigned to the hydrogen-bonded O H stretch of H 2 0 as a proton donor in the HOH.-N(3HP)
+
2176 The Journal of Physical Chemistry, Vol. 95, No. 7, I991
Person et al.
TABLE III: Calculated Frequencies, Intensities, and PotenHal Energy Distributions (PEDS) for the H20 Monomer, for the 3-Hydroxypyridine (3HP) Monomer, and for Their One-to-one Complexes"
Q, freq,b cm-' 3 I
PED^
vibrations of H 2 0 monomer 58 a OH st (100)
3730 (3733)C 3623 (3638)'
1627 ( I 590)r total H 2 0 int: 2
I
3659 (3626)
2
3029 (3061) 3016 (3040)
3
int,c km/mol
(58)' 18 (3) 107 (68) 183 ( 1 29)
s OH st (99)
HOH be (100)
vibrations of 3HP monomers OH st (100)
C4H st (52+), C5H st (40+) C4H st (44-), C6H st (28+), C5H st (27+) C6H st (64+), C5H St (32-)
freq,b cm-'
int: km/mol
PED^
vibrations of H 2 0 in HOH.-N(3HP) 134 f OH st (84+), (134)' bo OH st (i6-1 279 bo OH st (84+), anhc f OH str (16+)
3698 (2699) 3567 3427 (3404) 1644 (1 596)
(240) 86 HOH be (94) (40) 499 (419) vibrations of 3HP in HOH-.N(3HP) 92 OH st (100) 3659 3023
IO
3033
IO
3010
1
C4H s t (53-), C6H st (46+) C5H St (43-), C4H st (3O-), C6H st (26-) C5H St (55-5), C6H st (28+), C5H st (17+) C2H ~t (99-)
4
3001 (3029)
5
2970 (3001)
C2H str (99-)
2977
33
6
1612 (1591)
C3C4 St (26-), C4C5 st (17+), NlC2 st (13+), NlC6 St (l2-), C4H be (IO-) C2C3 ~t (24-), C5C6 st (21-)
1612
26
c 4 c 5 St (22-), C3C4 st ( 1 9+), NlC2 St (l6-), C4H be (IO+)
1605
12
C5H be (2l-), C2H be (18), NlC6 st (13+), C4C5 s t (l3-), C6H be (1 2-) C6H be (21+), NlC2 st (l7-), C4H be (l5-), C2C3 st (1 2-), C5C6 st (IO+)
1489
25
1438
156
C2C3 st (22-), C5C6 st (2O-), NlC6 st ( l l + ) , c 3 c 4 St ( I I + ) C5H be (21+), C2H be (l8-), C4C5 st (13+), NlC6 st (l2-), C6H be (1 I+) C6H be (22-), NlC2 st (13+), C4H be (14+), C2C3 st (12+), C5C6 st (IO-)
C2H be (35+), C6H be (27+), C4H be (14+), C5H be (12+), HOC be (IO+) C 3 0 st (48+), C2H be (15-)
1336
25
1266
91
HOC be (52+), C4H be ( I 8-), NlC2 St (12-) C6H be (24+), NlC2 st (18+), C2H be (15-), C5H be (14-), NlC6 st (IO+) NlC6 st (29-), C5H be (2l-), C4H be (14+), C2C3 St (1 3-) c 4 c 5 st (33+), C3C4 St ( I 7-), NlC6 st (17+), C4H be (16+)
1203
142
C5C6 St (58-), C5H be (IO-)
7
1603 ( 1 578)
8
1488 ( 1488)
9
1436 ( 1440)
IO
1336 ( 1 346)
II
1264 ( I 249)
12
13
1203 (1195) 1 I86
I I65
14
1096 (1 100)
15
I049 (1 022)
16
1018 (1011)
1 I88
51
1099
4
1051
26
1019
19
C2H be (34-), C6H be (27-), C4H be (15-), C5H be (l2-), HOC be (IO-) C30 st (48+), C2H be (15-), Ri de 1 (IO+) HOC be (51+), C4H be (l8-), NIC2 St (13-) C6H be (22-), NlC2 St (l8-), C2H be (16+), C5H be (14+), NlC6 str (IO-) NlC6 st (28+), C5H be (21+), C2C3 st (13+), C4H be (1 3-) c 4 c 5 st (34+), C4H be (1 7+), NlC6 st (16+), C3C4 St (16-) C5C6 St (55-), C5H be (IO-), NlC2 st (IO-)
int,b freq,' cm-I km/mol PEDd vibrations of H 2 0 in H20-H0(3HP) 3724 107 a OH st (100-1
(3724) 3623
(107)' 27
1619
113 (50) 247 (157) vibrations of 3HP in 3556 568 3416 antfa (568)'J (3425) 15 3024
s
OH st (99)
HOH be (100)
(1 590)
301 1
29
2996
12
2994
22
1612
14
1601 ( I 583)
30 (20)
1492
18
1438
198
( 1 443)
(100)
1345
34
1278 (1281)
28 (25)
1244 ( 1 254)
235 (55)
1187
IO
1loo
0.1
1050
29
1020
9
H20--H0(3HP) OH st (99) C4H st (54+), C5H st (38+) C4H st (42-), C5H st (29+), C6H st (28+) C2H st (56+), C6H st (24+), C5H st (l8-) C2H st (43-), C6H st (41+), C5H st (14-) C3C4 st (26-), C4C5 st (16+), NlC2 st (12+), NlC6 st (11-), C4H be (IO-) C2C3 st (22-), C5C6 st (21-) C5H be (19+), C2H be (l9-), C4C5 st (14+), NlC6 st (l4-), C6H be (12+) C6H be ( 1 7-), C4H be (l6+), NlC2 st (la+), C2C3 st (1 I+), C5C6 st (IO-), HOC be (IO+) C6H be (29+), C2H be (25+), HOC be (la+), C5H be (13+), C4H be (lo+) C30 st (39+), C2H be (24-), HOC be (11+) HOC be (35+), C4H be (2l-), C30 St (12-) C6H be (27-), N1C2 st (26-), C5H be (18+), C2H be (12+) NlC6 st (34+), C5H be (21+), C4H be (l5-), C2C3 st (12+) c 4 c 5 st (33-), C3C4 st (17+), C4H be (16-), N1C6 st (14-), N1C2 st (11+) C5C6 st (56+), C5H be (IO+)
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2777
Complexes of Water with 3-Hydroxypyridine
TABLE I11 (Continued) Q, freq,bcm-l 17
990
18
19 20
21
22
23 24 25 26 27 28 29 30
int,’ km/mol 0.2
PED^ C5H wa (57+), C6H wa (32-), C4H wa (27+) Ri de 1 (65+), C3C4 st (14+), C2C3 st (lo+) C6H wa (47+), C4H wa (46+), C2H wa (IO+) C2H wa (88+)
freq,” cm-’ 998 99 1
5
954
2
905
3
C5H wa (37+), C6H wa (21+), C4H wa (20-), C 3 0 wa ( I 5-) C30 st (22+), c 3 c 4 st (20+), Ri de 1 (l8-), Ri de 3 (16-), C2C3 st (12+) o Ri de 1 (97)
815
45
810
8
696
26
Ri de 2 (49-), Ri de 3 (32+) Ri de 3 (39+), Ri de 2 (35+), C30 st (12+) C30 wa (60+), o Ri de 3 (30-) o Ri de 2 (92-), o Ri 3 (19-) C30 be (83) HO tor (93-)
617
22
528
19
516
5
419
8
383 263
17 140
o Ri de 3 (54-), o Ri de 2 (18+), C30 wa (l4-), o Ri de I (IO+)
234
IO
total int: I” 2/‘
3” 4“
0 0 0 0
5”
0
int: km/mol 0.2
rotations and translations Rx Rz RY Tz
Tx
6’’
total low freq int: 465 total int = 465 + 988 + 183 = 1636
PEP C5H wa (53-), C6H wa- 143+1. __.. C4H wa i 2 k i ‘ Ri de 1 (65+), ’ c 3 c 4 st ( I S + )
freq,O cm-I 986
int,b km/mol 0.1
989
7
C4H wa (54-), C6H wa (44-)
94 1
6
C2H wa (94-)
918
2
C5H wa (39-), C4H wa (22+), C6H wa (l8-), C30 wa (1 5+) C 3 0 st (23+), c 3 c 4 st (20+), Ri de 1 (l7-), Ri de 3 (15-), C2C3 st (12+) o Ri de 1 (99)
810
40
810 (808)
7 (8)
700 (708) 609
29 (20)
530
6
519
1
419
2
404 637 (675) 237
IO 186 (60)
Ri de 2 (48+), Ri de 3 (31-) Ri de 3 (42-), Ri de 2 (30-), C30 st (12-) C30 wa (59-), o Ri de 3 (30+) o Ri de 2 (92-), o Ri de 3 (16-) C 3 0 be (83+) HO tor (89-) o Ri de 3 (54-),
5
1
o Ri de 2 (1 5+), C30 wa (l2-), HO tor (11+) L531 of H 2 0 monomer 549 200 273 132 117 2 137 134 47 38
7 2 471 2008
1805 vibrations involving both H20 and 3HP ipl OH-N be (57+) 192 11 OHN o wa (70+) 176 238 OH*-N st (108) 141 47 HO tor about 85 27 OH-N opl butterfly 56 22 opl butterfly 32 17 362 2167
PEDd C5H wa (55+), C6H wa (31-1. C4H wa (27+) Ri de 1 (as-), C3C4 st (13-), C2C3 8t (12-) C4H wa (38+), C6H wa (33+), C2H wa (29+) C2H wa (68-), C6H wa (19+), C4H wa (16+) C5H wa (36-), C6H wa (22-), C4H wa (19+), C 3 0 wa (16+) C 3 0 st (21-), c 3 c 4 st (203, Ri de 1 (18+), Ri de 3 (16+), C2C3 st (1 3-) o Ri de 1 (96) Ri de 2 (SO+), Ri de 3 (31-) Ri de 3 (41-), Ri de 2 (33-), c 3 0 st (1 1-) C 3 0 wa (60+), o Ri de 3 (29-) o Ri de 2 (92+), o Ride 3 (19+) C30 be (74-) OH tor, H2O o ro o Ri de 3 (57-), o Ri de 2 (17+), C30 wa (1 7-), o Ri de 1 (IO+) H 2 0 twist (Ry) H 2 0 wa OH-0 st [124) H 2 0 opl trans ipl butterfly opl butterfly
.For comparison the frequencies and intensities of the corresponding bands in the experimental spectra, when identified, are given in parentheses below the calculated values. Frequencies are scaled by a constant factor of 0.89. Experimental frequencies (in parentheses) correspond to those of the strongest band of several overlapping components, if such was observed for some vibrations. Experimental spectra were studied down to 500 cm only. eExperimental intensities (in parentheses) are expressed in relative units chosen so that the values marked by superscript/ fit the calculated intensities (see footnote b in Table 11). Here the total intensity of all overlapping bands is given if the experimental spectrum shows several components, as is found for some modes in Table 11. dAbbreviations are the same as used in Tables I and 11, with some additions as listed here: bo, hydrogen bonded; f, free (non-hydrogen-bonded); a, asymmetric; s, symmetric; 0, out of plane; ro, rocking; and tor, torsion. eFrquency of the band center for water monomer in an argon matrix as given in ref 31a. fThe integrated intensity of this mode is taken to be equal to the calculated intensity, providing the scaling factor for all the other bands within the spectrum from this particular molecule. #For hydrogen-bonded OH stretching modes the difference between the anharmonicities of the free and the hydrogen-bonded modes was subtracted from the calculated values to obtain the values listed as (u, anh) in the table. The anharmonicity of the free OH stretching modes of the monomer is (2x,uJNB = 160 cm-I and for the hydrogen-bonded OH stretch ( ~ x , u , ) ”is~ 300 cm-l; see text. complex. This assignment is supported by comparingdhese spectra with that for the pyridine-water complex (HOH-N(PY) where the corresponding band is observed at 3400 cm-I.” It is also very close to the band a t 3432 cm-’ for the HOH-.NH3 complex?s 3. The band at 3699 cm-I is assigned to the absorption from the free OH stretch of the proton donor HOH molecule in the complex to the N atom of the 3 H P molecule. This assignment is based on the assignment for the water dimer (where Q3(donor) is shifted to 3707 cm-I lo (or 3709 cm-’ in ref 31) and on the (30) Szczepaniak, K. Unpublished results.
calculated frequency for this band of the proton donor water molecule in other related complexes.32 4. We believe that the bands at (3724, 3721) cm-’ are the absorption by the asymmetric stretch (Q3(acceptor))of the H20 molecule as a proton acceptor in the H20.-H0(3HP) complex. (31) (a) Bentwood, R. M.;Barnes, A. J.; Orville-Thomas, W.J. J. Mol. Specrrosc. 1980,84, 391. (b) Ayers, G . P.; Pullin, Specrrochim. Acra 1976, 32A, 1629. (32) (a) Del Bene, J. E. Unpublished results. See also: (b) Frisch, M. J.; Poplc, J. A.; Del Bene, J. E. J . Phys. Chem. 1985,89, 3664; (c) Amos, R. D. Chem. Phys. 1986, 104, 145.
2778 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Person et al.
TABLE I V Calculated Frequencies, Intensities, and Potential Energy Distributions (PEDS) for the 3-Deuterioxypyridiw, 3HP(OD), Moaomer, Compared with the Infrared Spectrum for 3HP (OD) Isolated in an Argon Matrix at about 15 K
calculated values freq," int, Q, cm-I km/mol
PED^
1
54
2
2665 3029
3 4
3016 3001 2970
24 7 41
C4H St (44-), C6H st (28+), C5H S t (27+) C6H st (64-), C5H st (32+) C2H st (99-)
8
1610 1598 1483
32 7 63
C4C5 st (23-), C3C4 st (18+), NlC2 st (16-) C5C6 S t (21+), C2C3 st (21+), NlC6 st (13-), C3C4 st (13-) C5H be (25+), C2H be (l8-), NlC6 st (13-), C4C5 st (12+)
9
1427
84
C6H be (28-), NlC2 st (21+), C2C3 st (13+), C4H be (12+)
IO
1324 1258
IO 135
12 13 14
1189 1119
15 16
1020 991 989 943 908
6 22 28 7 IO
1
5
6 7
11
17 18 19
1055
20 21
904
22 23 24 25 26 27 28 29 30
799 697 609 523 514 418 363 239 189
810
11
OD st (loo-) C4H st (52+), C5H st (40+)
C2H be (45+), C6H be (22+), C4H be. (18+) C 3 0 st (49-), Ri de 1 (lo-), C4H be (IO-)
4 73
NlC2 st (I&), C6H be (27-), C5H be (20+), C2H be (lo+) NIC6 st (43-), C5H be (l6-), C4H be (15+) C4C5 st (38+), C4H be (19+), NlC2 st (15-), C3C4 st (12-) C5C6 st (56+), C5H be (IO+) Ri de 1 (63-), C2C3 st ( I S ) , C3C4 st (1 I-) C5H o wa (57+), C6H o wa (32-), C4H o wa (27+) C6H o wa (47+), C4H o wa (46+), C2H o wa (IO+) DOC be (77-), C2C3 st (IO+)
2 46
C2H o wa (88+) C5H o wa (37-), C6H o wa (2l-), C4H o wa (20+), C30 o wa (15+)
3 23
o Ri de 1 (97-)
0
5
6 1
4 15
16 68
experimental values freq,' int,d cm-I km/mol remarks* 2687 I MS 5d MS 2682 2677 OD st CH st 3064} 4 MS/FR (1591 + 1485) 3056 12 3035 CH st 3023 1 CH st 301 5 CH st 5 4 FR (1575 + 1434) 3006 2 FR (1575 + 1431) 3002 3 FR (1575 + 1426) 2996 55 1591 Ri st 1575 5 Ri st 1485 CH be., Ri st 43 1479 MS/FR (801 + 705) 18 1440 FR/MS (965 514') FR/MS (927 + 514') FR/MS (922 514*) 78 AR/MS (914 + 514') 1431 1426 Ri st, CH be 4 1325 CH be 1281 FR/MS (927 + 363') 1277) 32 FR/MS (922 + 363') 1273 FR/MS (919 + 363') 1268 20 FR/MS (1026 + 239*) 1263 50 co st 1241 FR/MS 2 X 620 21 3 FR/MS? 1235 1190 CH be, Ri st IO 4 1 I23 Ri st, CH be 14 1 IO3 Ri st 1026 Ri st 5 965 Ri de 1 3
C3C4 st (23+), C30 st (20+), Ri de 3 (17-), Ri de 1 (16-) Ri de 2 (52-), Ri de 3 (31+) Ri de 3 (40+), Ri de 2 (32+), C30 st (13+) C 3 0 o wa (a@), o Ri de 3 (30+) o Ri de 2 (92-), o Ri de 3 (19-) C 3 0 be (81+) o Ri de 3 (52+), C30 o wa (19+), o Ri de 2 ( 1 5 ) DO tor
+ +
?
927
17
-
41
CH o wa MS DOC be
803 801 -
41
MS CH o wa
705 620
38 4
? ?
1
o Ri de 1
Ri de 2
"Calculated frequencies have been scaled by a single scaling factor of 0.89. bAbbreviations: st, stretch; be, in plane bend; 0, out of plane; Ri, ring; de, deformation; wa, out of plane wag: for torsion. multicomponent bands observed in the experimental spectra, the frequency of the strongest component is underlined. 'Relative integrated intensities, given in such a way that the experimental absorbance of the OD stretching mode (labeled with the I ) is set equal to the calculated intensity. eAbbreviations: MS,matrix site effect splitting; FR, Fermi resonance, with some suggested combination modes. 'Reference intensity. 5. The absorption at 3626 cm-l (and that at 2677 cm-l) are due to the free O H (or OD) stretches of the non-hydrogen-bonded molecules of 3 H P (or 3HP(OD)). 6 . The splitting into doublets at (3724, 3721) cm-' and also at (3433, 3425) and (2537, 2534) cm-l may result from the rotational isomerism of the OH (and the O D group) in the 3 H P and 3HP(OD)). 7. Another possibility is that part of the absorption at (3724, 3721) cm-' comes from complex 111 mentioned above, the HOH-.O(H)(3HP) complex. It is also possible that the weak absorption band observed at 3424 cm-' (and/or 3414) cm-l which remains in HzO 3HP(OD) sample) might be due to the hydrogen-bonded stretching mode of the OH bond in H 2 0 acting as a proton donor to the O H group in 3 H P in the complex 111. These bands do not disappear after deuteration of the 3HP. If this alternate assignment (to complex 111) were correct, this would mean that the frequency for the hydrogen-bonded OH stretch of
+
H 2 0 in complex 111 (3424 or 3414 cm-I) would be much lower than the corresponding frequency (3575 cm-I) for the hydrogen-bonded OH stretch of the water dimer, implying a much stronger hydrogen bond in complex 111; this implication seems rather unlikely. Hence, the presence of complex I11 is very doubtful, as is the proposed tentative assignment (given above in this paragraph) of absorption bands related to complex 111. 8. The spectral pattern in the region 3450-3400 cm-' shown in Figure 3C, which is for a matrix with a high relative concentration of 3HP, is very similar to that in Figure 3D, from a matrix with a high relative concentration of H20. Hence we conclude that in both matrix samples only one-to-one complexes (both I and 11) are formed. Figure 4 shows the experimental spectra of the 3 H P system in the frequency region below 1700 cm-'. Figure 4, C and D, corresponds to argon matrices containing both 3 H P and H 2 0 , showing spectra of the complexes. Figure 4, A and B, shows the
Complexes of Water with 3-Hydroxypyridine
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2779
1
I
Figure 4. Comparison of the low-temperature (about 15 K) argon matrix IR spectra in the region from 1700 to 500 cm-’ of the noninteracting H20 (A) and 3HP (e) monomers with the corresponding IR argon matrix spectra (C) and (D) of mixtures of H20and 3HP at different relative concentrations. Absorption bands related to the complexes are marked with arrows. The assignment of all bands is discussed in the text and summarized in the tables.
spectra of the noninteracting monomers of 3 H P and H 2 0 . As can be seen in this figure the spectra of the samples containing complexes are almost identical with those from the isolated monomers. A few new bands related to the complexes are marked by arrows. These new bands appear to be assignable to the CO stretching and to the bending vibrations of the OH group perturbed by complex formation. The calculated frequencies, intensities, and potential energy distributions (PEDs) for the two complexes of 3 H P with HzO are compared in Table 111 with the calculated values for the isolated monomers and with the experimental values given in parentheses under the corresponding calculated values. The agreement is really very good. All of the bands that can be identified in the experimental spectrum are in very good agreement with the corresponding calculated values. Where no experimental values are given for comparison with the calculated values for the complexes, the bands for the complex are overlapped by the bands for the monomer, as predicted by the calculation. The results presented in Table Ill show that the frequencies and intensities of the vibrations of the H z O molecule when it acts as a proton donor in the HOH-aN(3HP) complex are relatively strongly perturbed. In contrast, the water vibrations for the HzO acting as a proton acceptor in the H20.-H0(3HP) complex are predicted to be relatively unaffected, but the OH stretch and bends (much less) of the hydroxyl group in the HO(3HP) molecule are relatively strongly changed. The remaining intramolecular vibrations of the 3 H P molecule in both complexes are predicted to be changed very little from the isolated molecule. Although the calculated scaled frequencies of the hydrogenbonded O H stretch in H20-.H0(3HP) and in HOH-.N(3HP) are predicted to be strongly shifted to lower frequencies in these complexes, they are still much higher than the observed frequencies. We believe that the explanation lies in the known fact that the anharmonicity of the hydrogen-bonded OH stretch is considerably greater than that of the nonbonded O H stretch (see the review by Sandorfy,zsband references therein). For methanol the anharmonicity correction ( ~ x , v , )of~ the ~ nonbonded OH stretching mode in the monomer is about 160 cm-’ while for hydrogen-bonded O H ( ~ X , V , )is~ 230 ~ cm-I. For the hydrogenbonded O H stretch in crystalline 3 H P we measure the value of ( Z X , V , ) ~ to ~ be about 300 c ~ - ’ , ~ O Hence, the calculated fre-
quencies of the hydrogen-bonded OH stretch should be corrected by subtracting the difference between the anharmonicity corrections for the free and bonded OH stretching modes (300 cm-’ (for solid 3HP) - 160 cm-* (for methanol) = 140 cm-I) in order to compare with the observed frequencies for the bonded OH stretch for the proton donor in the complex. We have assumed that this anharmonicity difference for the proton donor OH groups in both the HOH-N(3HP) and the H20--H0(3HP) complexes is the same. When this correction is applied, the calculated frequencies (marked anh) are in very close agreement with the experimentally observed frequencies (see Table 111). As can be seen in Table 111, the relatively large frequency shifts upon hydrogen-bond formation predicted for these stretching modes are consistent with the prediction of considerable intensification for these bands of the OH bond acting as the proton donor. We shall discuss this point further below, but for now call attention to the values given in Table 111 for the sums of the intensities for all the vibrations of the complexes, and how they change from the sums of the intensities for the monomers. It is particularly interesting to note that the intensities calculated for the low-frequency “intermolecular” modes in the complexes arise entirely from the calculated intensity for the pure (zero frequency) rotational modes of the H 2 0 monomer. The comparison of the vibrations of the HzO monomer with the vibrations of H 2 0 in the HOH-.N(3HP) complex where it is the proton donor deserves further comment. As has been mentioned earlier, the two OH bonds are no longer equivalent for this complex, as can be seen in Table I11 in the PEDs for 4 3 and Q1 compared with those for the same modes in the monomer. In the monomer both modes have equal contributions from stretching each OH bond (Q3 out of phase (a), and Q1 in phase (s)). In complexes, where H20acts as a proton donor through one of the O H bonds, the normal-mode description changes, so that the high-frequency mode (still called 4 3 in the Table 111) is now primarily the motion of the free (nonbonded) OH group (86% in PED) while the low-frequency mode (still called Q1) is primarily the motion of the bonded hydrogen atom from the OH group acting as the proton donor (89% in PED). In each case the remaining 16% (1 1%) contribution in the PED for each mode is from the other OH bond. This means that a simple comparison of the “shift” of frequencies or intensities is an oversimplification
2780 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 TABLE V Changes in the Calculated Bond Properties of OH Groups Acting as Proton Donors in Some Hydrogen-Bonded Complexes (SpeciflcrUy for HOH-OHB H20.-HO(3HP), and for HOH.-N( 3HP))‘ H0H.a. H20. HOH... property 3HP H 2 0 N(3HP) HO(3HP) H20 fOH’ 7.482 7.558 7.203 7.206 7.304 AfOHb -0.355 -0.276 -0.254 0.41 0.18 0.68 0.75 0.50 PYY +0.50 +0.34 +0.32 APrY” 0.79 0.50 3x / m 0.37 0.27 0.65 A(3~’/m)~ +0.38 +0.42 +0.23 P 0.33 0.30 0.42 0.46 0.40 AP +0.12 +0.13 +0.10 ’Properties presented here include the bond force constant foH in aJ A-2, the atomic polar tensor element Pyyin e (1 e = 1.602 X C), and the invariants (3x2/m) and fi for the proton donor H atom (they axis is chosen parallel to the approximately linear OH-0 bond). bThese force constants are the diagonal force constants multiplying (ArOH)’ in the potential energy, so they are the harmonic force constants from the calculation, scaled by (0.89)2 to be consistent with the scaled values of the frequencies. cThese values are the differences between the property for the H atom in the complex where the H is the proton donor and the value for the nonbonded molecule.
of what actually happens when a complex forms. In order to examine the chemically significant changes that occur during the process of hydrogen-bond formation, it is much more desirable to determine the changes in the force constant of the proton donor O H bond than to calculate just the ”frequency shift”. Similarly, the changes in the polar properties (for example, the atomic polar tensors of this bond are much more significant than are the “intensifications” of the O H stretching modes. The analysis of the latter has been examined in detail for the water dimer by Zilles and Person.34 The problems of choosing the axes used to describe the APTs are discussed there, and it is shown that if the axes for the APTs of the proton donor are chosen so that t h e y axis is parallel to the O H - 0 hydrogen bond, the only element of the polar tensors for atoms in either water molecule that changes from the free molecule is the Pyy element of the H atom acting as the proton donor (and other elements that are influenced by this one element). The changes in the properties of the O H bonds acting as proton donors in the two complexes studied here are given in Table V, which also includes similar results for the water dimer. We see that the changes calculated for the harmonic force constants for stretching the proton donor O H bond are quite similar for all three OH-X complexes (here X is the proton acceptor), with the change for the water dimer somewhat less than for the stronger complexes. Also the changes in the P polar tensor for the H atom are similar. Because the magnitugs of the polar tensor elements depend strongly on the choice of the Cartesian axes used to express them, it is useful to examine changes in some of the invariants of the AFTs (see refs 33-35] for definitions and further discussion). We (33) (a) Biarge, J. F.; Henanz, J.; Morcillo, J. An. R. Soc. ESP.Fis. Quim. 1961, A57, 81. (b) See also: Person, W. B. In Vibrational Intensities in Infrared and Roman Spectroscopy; Person, W . B., Zerbi, G., Eds.; Elsevier: Amsterdam, 1982; Chapters 4 and 14. (34) Zilles, B. A.; Person, W. B. J . Chem. Phys. 1983, 79, 65.
Person et al. show in Table V values for the mass-weighted squared effective charges (3x2/m) and for the mean dipole derivative (p). The changes in these properties are also quite similar for the H atom in the OH proton donor bond for the three OH.-X complexes shown here, although there is clearly a trend both in AfoH and in the changes in the intensity parameters from the weaker water dimer to the stronger complexes I and 11. Zilles and Person” suggested that all the changes in the infrared intensities of the fundamental modes in H,O that occur when the term of the APT dimer is formed are due to the changes in the PYy of the H atom that is acting as the proton donor. We can extend that hypothesis to the two complexes of H 2 0 with 3 H P by examining the results given for the intensity parameters in Table V. The sum of the intensities for all the viprations of the complexes is given by the sum over all the atoms of the mass-weighted mean-squared effective charges (seeref 35 and references therein) multiplied by the conversion factor (974.9) to convert to intensities in km mol-’. If we assume that the only mass-weighted squared effective charge which changes upon complex formation is the one for the H atom acting as a proton donor, then the intensity sum for the HOH-N(3HP) complex, for example, is expected to be greater than the intensity sum for the 3 H P and H 2 0 monomers (including the sum of rotational intensities especially for the latter) by 0.38 X 974.9 = 370 km mol-’, in perfect agreement with the difference in the intensity sums shown in Table 111 (2008 - 1636 = 372 km mol-’). Relative Concentrations of Complexes
Even though there are considerable errors in intensity measurements and in calculations of absolute intensities for these molecules, it is still worthwhile to try to estimate the relative concentrations of the species present in the matrix spectra of Figures 3 and 4. We can estimate the concentration of the HOH.-N(3HP) complex relative to that of the H20-.H0(3HP) complex by measuring the integrated absorbance ((IHBI) of the hydrogen-bonded OH stretch of complex I relative to that of the hydrogen-bonded OH stretch (aHB11) in complex 11. The relative concentrations can then be estimated by using the formula [HOH**N(3HP)]/ [H,O**H0(3HP)] = (aHBI/aHBII)(AHBII/AHBl)
where A H B i are the calculated values of the integrated molar absorption coefficients given in Table 111. The ratio of the concentrations of the complexes estimated from this formula is [HOH...N(3HP)]/[H20.-H0(3HP)] = 1.28 0.08. Hence, we conclude that the two complexes have similar stabilities. The calculated stabilities for these complexes at I5 K from MP2/6-3 1+G(d,p) electronic energies and HF/6-31G(d) vibrational frequencies are -7.4 and -5.3 kcal/mol for H20-H0(3HP) and HOH-N(3HP), respectively. Both complexes are thus more stable than the water dimer, which has a stabilization energy of -4.2 kcal/mol.
*
Acknowledgment. We are grateful for support of the experimental work from N I H Research Grant No. 32988. These calculations were carried out on the Cray Y-MP8/864 at the Ohio Supercomputing Center. Registry No. 3HP, 109-00-2; H20, 7732-18-5. (35) Person, W. B.; KuBulat, K. J . Mol. Struct. 1988, 173, 357.
