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Conformational equilibrium in 3-hydroxy-pyridine
Chemical Physics Letters 435 (2007) 10–13 www.elsevier.com/locate/cplett
Conformational equilibrium in 3-hydroxy-pyridine Raquel Sanchez 1, B. Michela Giuliano, Sonia Melandri, Walther Caminati
*
Dipartimento di Chimica ‘G. Ciamician’ dell’Universita`, Via Selmi 2, I-40126 Bologna, Italy Received 16 November 2006; in final form 6 December 2006 Available online 19 December 2006
Abstract The rotational spectrum of 3-hydroxy-pyridine has been investigated by free-jet absorption millimeter-wave spectroscopy. Two conformers (cis and trans) related to the orientation of the hydroxyl group with respect to the nitrogen atom, have been observed. The cis form is more stable by 380 (1 0 0) cm1. 2006 Elsevier B.V. All rights reserved.
1. Introduction 2-Hydroxy-, 3-hydroxy- and 4-hydroxy-pyridine have the same functional groups and similar chemical properties, which include the possibility of keto-enolic tautomerism. However, the features of their rotational spectra are entirely different. 2-Hydroxy-pyridine has been widely investigated, since it is considered as a prototype molecule for the study of keto-enolic tautomerism in aromatic heterocyclic compounds [1–4]. Its rotational spectrum contains information on both 2-hydroxy-pyridine and 2-pyridinone tautomers, with the enolic form being more stable by 0.32(3) kJ/mol [4]. The rotational spectrum of 4-hydroxypyridine displayed only the enolic form. Due to its symmetry, the internal rotation of the hydroxyl group connects two equivalent minima, producing a systematic even splitting of the lc-type transitions; from this splitting the barrier to internal rotation of the OH group was determined [5]. No spectroscopic or theoretical investigations have been reported for the third isomer of hydroxy-pyridine, 3hydroxy-pyridine (3HP). We will show here that its rotational spectrum presents different features with respect to
*
Corresponding author. Fax: +39 051 2099456. E-mail address: [email protected] (W. Caminati). 1 Permanent address: Grupo de Espectroscopı´a Molecular (GEM), Departamento de Quı´mica Fı´sica y Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain. 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.048
those of the other two isomers, consistent in the detection of two different conformers (cis and trans) shown in Fig. 1. 2. Experimental The Stark modulated free-jet absorption millimeterwave spectrometer used in this study has already been described elsewhere [6]. It covers the frequency range 59.5–78.3 GHz and the accuracy of the frequency measurements is ca. 50 kHz. Argon at a pressure of 200 mbar was flown through a cuvette with 3HP heated to 160 C and the mixture was then expanded to about 5 · 103 mbar through a nozzle with a diameter of 0.3 mm. 3HP was purchased from Aldrich and used without further purifications. The deuterated species were formed flowing the carrier gas over a sample of D2O (99.9%, purchased from Cambridge Isotope Laboratories) at room temperature, and subsequently over the heated 3HP sample. 3. Theoretical calculations We performed preliminary density functional calculations at the B3LYP/6-31G(d,p) level of approximation, in order to obtain information on the relative energies of the two conformers, and their reference starting geometries. The GAUSSIAN03 software package has been used [7]. In addition, the calculations provide rotational and
R. Sanchez et al. / Chemical Physics Letters 435 (2007) 10–13
11
Some of the lb-R-type lines show a resolvable hyperfine quadrupole structure because of the 14N nuclear spin I = 1. All transitions, given as supplementary material, have been fitted simultaneously with Watson Hamiltonian (S-reduction, Ir-representation) [8]. The obtained spectroscopic constants and Pcc values are reported in Table 2 for both conformers. 5. Conformations, structures and relative energies of the two conformers of 3HP Fig. 1. The two conformers of 3HP.
quadrupole coupling constants, and the components of the dipole moment along the principal axes. All these data, useful for microwave spectroscopy studies, are reported in Table 1. The trans form is calculated to be slightly higher in energy, ca. 200 cm1, so that the rotational spectra of both species are likely to be observed with our spectrometer. We also calculated the barrier to the conformational interconversion, which resulted to be 1452 cm1 with respect to the cis conformer. 4. Rotational spectrum The first trial calculations of the spectra have been based on the rotational constants of Table 1. The recorded spectrum exhibits a typical pattern where the rotational transitions for the cis and trans conformer are closely spaced (the difference in frequency for the corresponding transition is in the range 10–20 MHz) and display an intensity ratio of 2:1 in favor of the cis conformer. Ten la-R-type transitions were first assigned, for both conformers, with J = 13 and K1 ranging from 5 to 10. Subsequently 20 more la-R-type lines were measured together with 29 lines of lb-R-type giving a total number of 59 fitted transitions for the cis conformer and 60 for the trans species. The spectra for the deuterated species were assigned in a similar way, with a total number of 48 and 22 transitions for the cis and trans conformers, respectively. Table 1 B3LYP/6-31G(d,p) calculations of 3-OH-pyridine
A/MHz B/MHz C/MHz la/D lb/D vaa/MHz vbb/MHz vcc/MHz vab/MHz Econf/cm1 a
cis
trans
5818 2684 1837 1.2 0.6 0.12 3.43 3.30 2.80 0a
5816 2684 1836 1.3 3.2 0.07 3.45 3.37 2.82 203
Absolute energy: 323.51230 Ha.
The planar moment of inertia Pcc, has practically the same value for the most abundant (O–H) and O–D species ˚ 2, see Table of the two conformers (from 0.048 to 0.108 u A 2), in accord with their planarity. The discrimination between the two conformers from the values of the rotational constants is not possible, because they are – especially for the OH species – very similar for the two species. However, it has been possible to establish the orientation of the hydroxyl hydrogen for the two species from their substitution coordinates [9]. They are reported in Table 3, where one can note significant differences between the jbj coordinates of the trans and cis conformers. The conformational composition in the vapor prior to expansion can remain basically unaltered if conformational relaxation is prevented by a high energy barrier to interconversion and if a large motion takes place. It has been shown in experimental work that the upper limit of the barrier for which relaxation takes place in Ar is higher than twice kT [10]. In our case 2kT is ca. 600 cm1, much lower than the barrier to interconversion. In addition the OH group should undergo a large rotation (180) in going from one conformer to the other. We can then assume that no conformational relaxation takes place during the supersonic Table 2 Spectroscopic constants of 3HP (Watson’s S-reduction and Irrepresentation) cis
trans
O–H A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz vaa/MHz (vbb vbb) MHz ˚2 Pcc/uA r/MHze Nf a b c d e f
a
5817.073(5) 2689.664(3) 1839.407(3) 0.102(7) 0.42(1) 0.6(1) 0.049(3) [0.00]c [0.12]d 7.3(2) 0.048 0.05 59
O–D
O–H
O–D
5768.978(6) 2596.117(4) 1790.679(5) 0.11(1) 0.37(1) 0.5(1) 0.042(5) [0.00]c [0.12]d 7.6(3) 0.086 0.05 48
5815.847(5) 2689.111(3) 1839.070(4) 0.114(8) 0.40(1) 0.7(1) 0.038(3) 0.018(4) [0.07]d 7.5(2) 0.062 0.05 60
5777.237(3) 2591.486(9) 1789.34(1) 0.19(4) [0.40]b [0.7]b [0.038]b [0.018]b [0.07]d 7.0(3) 0.108 0.06 22
Errors in parenthesis are expressed in units of the last digit. Fixed at the value of the corresponding most abundant species. Undetermined from the fit: fixed at zero. Undetermined from the fit: fixed at the small ab initio value. Standard deviation of the fit. Number of transitions in the fit.
12
R. Sanchez et al. / Chemical Physics Letters 435 (2007) 10–13
Table 3 rs Coordinates of of the hydroxyl hydrogens in the principal axis systems of the cis and trans conformers of 3HP, respectively cis ˚b jaj/A ˚ jbj/A ˚ jcj/A a b
trans
Exp.
Calc.a
Exp.
Calc.a
2.5982 (6) 0.881 (1) 0.0
2.6088 0.885 0.0
2.6588 (6) 0.790 (2) 0.0
2.6711 0.795 0.0
From the B3LYP/6-31G(d,p) structures (see Table 4). Calculated coordinates are signed.
Table 4 Calculated structure (B3LYP/6-31G**) of 3HP ˚) Bond distances (A Valence angles ()
N1–C2 C2–C3 C3–C4 C4–C5 C5–C6 N1–C6 C3–O O–H C2–H C4–H C5–H C6–H
cis
trans
1.335 1.401 1.397 1.391 1.397 1.338 1.363 0.967 1.092 1.085 1.086 1.087
1.333 1.403 1.395 1.394 1.394 1.340 1.364 0.966 1.088 1.088 1.086 1.088
N1C2C3 C2C3C4 C3C4C5 C4C5C6 C5C6N1 C6N1C2 OC3C4 HOC3 HC2N HC4C5 HC5C4 HC6C5
cis
trans
117.8 118.4 118.1 119.2 123.0 117.8 118.6 109.2 116.7 122.2 120.5 120.7
118.0 118.5 118.3 118.9 123.0 118.0 123.9 109.5 117.8 121.0 120.6 120.6
The dihedral angle C2C3-OH is 0 and 180 for the cis and trans conformers, respectively.
expansion, and obtain some information on the conformational equilibrium. To assess this we performed a series of relative intensity measurements. First we measured the relative intensities within the line manifold of each conformer in order to estimate the reached ‘rotational temperature’. We realized that the effective rotational temperature was in the usual range of our spectrometer [6], ca. 10 K for both conformers. Assuming that the ‘conformational temperature’ (Tconf) remained at the sample temperature prior to the supersonic expansion (that is about 160 C), we calculated the conformational ground state energy differences, (E0.0)C–T = (E0.0)C (E0.0)T, applying the following equation which is derived from the rotational line intensity equation [11] combined with the Boltzmann distribution applied to the rotational and conformational populations: ðDE0:0 ÞCT ¼ k T conf
ble 1). In the present case Tconf = 433 and Trot = 10 K. We obtained (E0.0)CT = 380 ± 100 cm1, slightly larger than the value obtained from the B3LYP/6-31G(d,p) calculations. The error is mainly attributable to the uncertainty on the theoretical la,G values, which can be up to 0.3 D. The B3LYP/6-31G(d,p) geometries, reported in Table 4 for both species, reproduce the experimental rotational constants very satisfactorily, within 5 MHz, so that we did not need to try any structural improvement. The main structural difference between the two conformers is found in the OC3C4 valence angle (118.6 and 123.9 for the cis and trans forms, respectively), presumably due to the steric hindrance between the OH and C4 hydrogens in the trans configuration. 6. Conclusions 3HP, the third and final isomer of hydroxy-pyridine studied by rotational spectroscopy, is characterized by the existence of two conformers with a relatively small energy difference. The cis conformer is more stable by ca. 300 cm1, in agreement with the theoretical value. In featuring this conformational equilibrium, 3HP is markedly different with respect to 2-hydroxy-pyridine/2pyridinone, a prototype for the study of tautomeric equilibrium, and 4-hydroxy-pyridine, where the internal rotation of the phenolic OH connects two equivalent conformations. The present investigation together with the preceding studies on the hydroxy-pyridine isomers contributes to the understanding of intramolecular interactions with the OH groups in heterocyclic rings in relation to their tautomeric, conformational and tunnelling properties. Acknowledgements The authors thank Mr. Aldo Millemaggi for technical support. Acknowledgements are also due to the Universita` degli Studi di Bologna and to the Ministero dell’Istruzione, dell’Universita` e della Ricerca. R.S. gratefully acknowledges an FPI grant from the Spanish Ministerio de Ciencia y Tecnologı´a. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett. 2006.12.048.
ln½ðI T xC la;C cC m2C Þ=ðI C xT la;T cT m2T Þ þ ½ðErot ÞC ðErot ÞT T conf =T rot
ð1Þ
where the subscripts 0,0 stand for v = 0 and J = 0, and C, T for cis and trans, respectively. I is the absorption signal heigh, x the conformational degeneracy, la the a-dipole moment component, and c and m the line strength and frequency, respectively. We used only a-type transitions because the ab initio la-dipole moment components are relatively large and similar for the two conformers (see Ta-
References [1] M.R. Nimlos, D.F. Kelley, E.R. Bernstein, J. Phys. Chem. 93 (1989) 643. [2] A. Held, B.B. Champagne, D.W. Pratt, J. Chem. Phys. 95 (1991) 8732. [3] L.D. Haterley et al., J. Phys. Chem. 97 (1993) 46. [4] A. Fu, H. Li, D. Du, Z. Zhou, J. Phys. Chem. A 109 (2005) 1468. [5] R. Sanchez, B.M. Giuliano, S. Melandri, W. Caminati, Chem. Phys. Lett. 425 (2006) 6.
R. Sanchez et al. / Chemical Physics Letters 435 (2007) 10–13 [6] S. Melandri, W. Caminati, L.B. Favero, A. Millemaggi, P.G. Favero, J. Mol. Struct. 352/353 (1995) 253. [7] M.J. Frisch et al., GAUSSIAN03, Revision B.04, Gaussian Inc., Pittsburgh, PA, 2003. [8] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 6, Elsevier, Amsterdam, 1977, p. 1.
13
[9] J. Kraitchman, Am. J. Phys. 21 (1953) 17. [10] R.S. Ruoff, T.D. Klots, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 93 (1990) 3142. [11] W. Gordy, L.R. Cook, Microwave Molecular Spectra, third ed., in: A. Weissberger (Ed.), Techniques of Chemistry, vol. XVIII, John Wiley & Sons Inc., New York, 1984.
Conformational equilibrium in 3-hydroxy-pyridine Raquel Sanchez 1, B. Michela Giuliano, Sonia Melandri, Walther Caminati
*
Dipartimento di Chimica ‘G. Ciamician’ dell’Universita`, Via Selmi 2, I-40126 Bologna, Italy Received 16 November 2006; in final form 6 December 2006 Available online 19 December 2006
Abstract The rotational spectrum of 3-hydroxy-pyridine has been investigated by free-jet absorption millimeter-wave spectroscopy. Two conformers (cis and trans) related to the orientation of the hydroxyl group with respect to the nitrogen atom, have been observed. The cis form is more stable by 380 (1 0 0) cm1. 2006 Elsevier B.V. All rights reserved.
1. Introduction 2-Hydroxy-, 3-hydroxy- and 4-hydroxy-pyridine have the same functional groups and similar chemical properties, which include the possibility of keto-enolic tautomerism. However, the features of their rotational spectra are entirely different. 2-Hydroxy-pyridine has been widely investigated, since it is considered as a prototype molecule for the study of keto-enolic tautomerism in aromatic heterocyclic compounds [1–4]. Its rotational spectrum contains information on both 2-hydroxy-pyridine and 2-pyridinone tautomers, with the enolic form being more stable by 0.32(3) kJ/mol [4]. The rotational spectrum of 4-hydroxypyridine displayed only the enolic form. Due to its symmetry, the internal rotation of the hydroxyl group connects two equivalent minima, producing a systematic even splitting of the lc-type transitions; from this splitting the barrier to internal rotation of the OH group was determined [5]. No spectroscopic or theoretical investigations have been reported for the third isomer of hydroxy-pyridine, 3hydroxy-pyridine (3HP). We will show here that its rotational spectrum presents different features with respect to
*
Corresponding author. Fax: +39 051 2099456. E-mail address: [email protected] (W. Caminati). 1 Permanent address: Grupo de Espectroscopı´a Molecular (GEM), Departamento de Quı´mica Fı´sica y Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain. 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.048
those of the other two isomers, consistent in the detection of two different conformers (cis and trans) shown in Fig. 1. 2. Experimental The Stark modulated free-jet absorption millimeterwave spectrometer used in this study has already been described elsewhere [6]. It covers the frequency range 59.5–78.3 GHz and the accuracy of the frequency measurements is ca. 50 kHz. Argon at a pressure of 200 mbar was flown through a cuvette with 3HP heated to 160 C and the mixture was then expanded to about 5 · 103 mbar through a nozzle with a diameter of 0.3 mm. 3HP was purchased from Aldrich and used without further purifications. The deuterated species were formed flowing the carrier gas over a sample of D2O (99.9%, purchased from Cambridge Isotope Laboratories) at room temperature, and subsequently over the heated 3HP sample. 3. Theoretical calculations We performed preliminary density functional calculations at the B3LYP/6-31G(d,p) level of approximation, in order to obtain information on the relative energies of the two conformers, and their reference starting geometries. The GAUSSIAN03 software package has been used [7]. In addition, the calculations provide rotational and
R. Sanchez et al. / Chemical Physics Letters 435 (2007) 10–13
11
Some of the lb-R-type lines show a resolvable hyperfine quadrupole structure because of the 14N nuclear spin I = 1. All transitions, given as supplementary material, have been fitted simultaneously with Watson Hamiltonian (S-reduction, Ir-representation) [8]. The obtained spectroscopic constants and Pcc values are reported in Table 2 for both conformers. 5. Conformations, structures and relative energies of the two conformers of 3HP Fig. 1. The two conformers of 3HP.
quadrupole coupling constants, and the components of the dipole moment along the principal axes. All these data, useful for microwave spectroscopy studies, are reported in Table 1. The trans form is calculated to be slightly higher in energy, ca. 200 cm1, so that the rotational spectra of both species are likely to be observed with our spectrometer. We also calculated the barrier to the conformational interconversion, which resulted to be 1452 cm1 with respect to the cis conformer. 4. Rotational spectrum The first trial calculations of the spectra have been based on the rotational constants of Table 1. The recorded spectrum exhibits a typical pattern where the rotational transitions for the cis and trans conformer are closely spaced (the difference in frequency for the corresponding transition is in the range 10–20 MHz) and display an intensity ratio of 2:1 in favor of the cis conformer. Ten la-R-type transitions were first assigned, for both conformers, with J = 13 and K1 ranging from 5 to 10. Subsequently 20 more la-R-type lines were measured together with 29 lines of lb-R-type giving a total number of 59 fitted transitions for the cis conformer and 60 for the trans species. The spectra for the deuterated species were assigned in a similar way, with a total number of 48 and 22 transitions for the cis and trans conformers, respectively. Table 1 B3LYP/6-31G(d,p) calculations of 3-OH-pyridine
A/MHz B/MHz C/MHz la/D lb/D vaa/MHz vbb/MHz vcc/MHz vab/MHz Econf/cm1 a
cis
trans
5818 2684 1837 1.2 0.6 0.12 3.43 3.30 2.80 0a
5816 2684 1836 1.3 3.2 0.07 3.45 3.37 2.82 203
Absolute energy: 323.51230 Ha.
The planar moment of inertia Pcc, has practically the same value for the most abundant (O–H) and O–D species ˚ 2, see Table of the two conformers (from 0.048 to 0.108 u A 2), in accord with their planarity. The discrimination between the two conformers from the values of the rotational constants is not possible, because they are – especially for the OH species – very similar for the two species. However, it has been possible to establish the orientation of the hydroxyl hydrogen for the two species from their substitution coordinates [9]. They are reported in Table 3, where one can note significant differences between the jbj coordinates of the trans and cis conformers. The conformational composition in the vapor prior to expansion can remain basically unaltered if conformational relaxation is prevented by a high energy barrier to interconversion and if a large motion takes place. It has been shown in experimental work that the upper limit of the barrier for which relaxation takes place in Ar is higher than twice kT [10]. In our case 2kT is ca. 600 cm1, much lower than the barrier to interconversion. In addition the OH group should undergo a large rotation (180) in going from one conformer to the other. We can then assume that no conformational relaxation takes place during the supersonic Table 2 Spectroscopic constants of 3HP (Watson’s S-reduction and Irrepresentation) cis
trans
O–H A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz vaa/MHz (vbb vbb) MHz ˚2 Pcc/uA r/MHze Nf a b c d e f
a
5817.073(5) 2689.664(3) 1839.407(3) 0.102(7) 0.42(1) 0.6(1) 0.049(3) [0.00]c [0.12]d 7.3(2) 0.048 0.05 59
O–D
O–H
O–D
5768.978(6) 2596.117(4) 1790.679(5) 0.11(1) 0.37(1) 0.5(1) 0.042(5) [0.00]c [0.12]d 7.6(3) 0.086 0.05 48
5815.847(5) 2689.111(3) 1839.070(4) 0.114(8) 0.40(1) 0.7(1) 0.038(3) 0.018(4) [0.07]d 7.5(2) 0.062 0.05 60
5777.237(3) 2591.486(9) 1789.34(1) 0.19(4) [0.40]b [0.7]b [0.038]b [0.018]b [0.07]d 7.0(3) 0.108 0.06 22
Errors in parenthesis are expressed in units of the last digit. Fixed at the value of the corresponding most abundant species. Undetermined from the fit: fixed at zero. Undetermined from the fit: fixed at the small ab initio value. Standard deviation of the fit. Number of transitions in the fit.
12
R. Sanchez et al. / Chemical Physics Letters 435 (2007) 10–13
Table 3 rs Coordinates of of the hydroxyl hydrogens in the principal axis systems of the cis and trans conformers of 3HP, respectively cis ˚b jaj/A ˚ jbj/A ˚ jcj/A a b
trans
Exp.
Calc.a
Exp.
Calc.a
2.5982 (6) 0.881 (1) 0.0
2.6088 0.885 0.0
2.6588 (6) 0.790 (2) 0.0
2.6711 0.795 0.0
From the B3LYP/6-31G(d,p) structures (see Table 4). Calculated coordinates are signed.
Table 4 Calculated structure (B3LYP/6-31G**) of 3HP ˚) Bond distances (A Valence angles ()
N1–C2 C2–C3 C3–C4 C4–C5 C5–C6 N1–C6 C3–O O–H C2–H C4–H C5–H C6–H
cis
trans
1.335 1.401 1.397 1.391 1.397 1.338 1.363 0.967 1.092 1.085 1.086 1.087
1.333 1.403 1.395 1.394 1.394 1.340 1.364 0.966 1.088 1.088 1.086 1.088
N1C2C3 C2C3C4 C3C4C5 C4C5C6 C5C6N1 C6N1C2 OC3C4 HOC3 HC2N HC4C5 HC5C4 HC6C5
cis
trans
117.8 118.4 118.1 119.2 123.0 117.8 118.6 109.2 116.7 122.2 120.5 120.7
118.0 118.5 118.3 118.9 123.0 118.0 123.9 109.5 117.8 121.0 120.6 120.6
The dihedral angle C2C3-OH is 0 and 180 for the cis and trans conformers, respectively.
expansion, and obtain some information on the conformational equilibrium. To assess this we performed a series of relative intensity measurements. First we measured the relative intensities within the line manifold of each conformer in order to estimate the reached ‘rotational temperature’. We realized that the effective rotational temperature was in the usual range of our spectrometer [6], ca. 10 K for both conformers. Assuming that the ‘conformational temperature’ (Tconf) remained at the sample temperature prior to the supersonic expansion (that is about 160 C), we calculated the conformational ground state energy differences, (E0.0)C–T = (E0.0)C (E0.0)T, applying the following equation which is derived from the rotational line intensity equation [11] combined with the Boltzmann distribution applied to the rotational and conformational populations: ðDE0:0 ÞCT ¼ k T conf
ble 1). In the present case Tconf = 433 and Trot = 10 K. We obtained (E0.0)CT = 380 ± 100 cm1, slightly larger than the value obtained from the B3LYP/6-31G(d,p) calculations. The error is mainly attributable to the uncertainty on the theoretical la,G values, which can be up to 0.3 D. The B3LYP/6-31G(d,p) geometries, reported in Table 4 for both species, reproduce the experimental rotational constants very satisfactorily, within 5 MHz, so that we did not need to try any structural improvement. The main structural difference between the two conformers is found in the OC3C4 valence angle (118.6 and 123.9 for the cis and trans forms, respectively), presumably due to the steric hindrance between the OH and C4 hydrogens in the trans configuration. 6. Conclusions 3HP, the third and final isomer of hydroxy-pyridine studied by rotational spectroscopy, is characterized by the existence of two conformers with a relatively small energy difference. The cis conformer is more stable by ca. 300 cm1, in agreement with the theoretical value. In featuring this conformational equilibrium, 3HP is markedly different with respect to 2-hydroxy-pyridine/2pyridinone, a prototype for the study of tautomeric equilibrium, and 4-hydroxy-pyridine, where the internal rotation of the phenolic OH connects two equivalent conformations. The present investigation together with the preceding studies on the hydroxy-pyridine isomers contributes to the understanding of intramolecular interactions with the OH groups in heterocyclic rings in relation to their tautomeric, conformational and tunnelling properties. Acknowledgements The authors thank Mr. Aldo Millemaggi for technical support. Acknowledgements are also due to the Universita` degli Studi di Bologna and to the Ministero dell’Istruzione, dell’Universita` e della Ricerca. R.S. gratefully acknowledges an FPI grant from the Spanish Ministerio de Ciencia y Tecnologı´a. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett. 2006.12.048.
ln½ðI T xC la;C cC m2C Þ=ðI C xT la;T cT m2T Þ þ ½ðErot ÞC ðErot ÞT T conf =T rot
ð1Þ
where the subscripts 0,0 stand for v = 0 and J = 0, and C, T for cis and trans, respectively. I is the absorption signal heigh, x the conformational degeneracy, la the a-dipole moment component, and c and m the line strength and frequency, respectively. We used only a-type transitions because the ab initio la-dipole moment components are relatively large and similar for the two conformers (see Ta-
References [1] M.R. Nimlos, D.F. Kelley, E.R. Bernstein, J. Phys. Chem. 93 (1989) 643. [2] A. Held, B.B. Champagne, D.W. Pratt, J. Chem. Phys. 95 (1991) 8732. [3] L.D. Haterley et al., J. Phys. Chem. 97 (1993) 46. [4] A. Fu, H. Li, D. Du, Z. Zhou, J. Phys. Chem. A 109 (2005) 1468. [5] R. Sanchez, B.M. Giuliano, S. Melandri, W. Caminati, Chem. Phys. Lett. 425 (2006) 6.
R. Sanchez et al. / Chemical Physics Letters 435 (2007) 10–13 [6] S. Melandri, W. Caminati, L.B. Favero, A. Millemaggi, P.G. Favero, J. Mol. Struct. 352/353 (1995) 253. [7] M.J. Frisch et al., GAUSSIAN03, Revision B.04, Gaussian Inc., Pittsburgh, PA, 2003. [8] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 6, Elsevier, Amsterdam, 1977, p. 1.
13
[9] J. Kraitchman, Am. J. Phys. 21 (1953) 17. [10] R.S. Ruoff, T.D. Klots, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 93 (1990) 3142. [11] W. Gordy, L.R. Cook, Microwave Molecular Spectra, third ed., in: A. Weissberger (Ed.), Techniques of Chemistry, vol. XVIII, John Wiley & Sons Inc., New York, 1984.