3119
J. Phys. Chem. 1991, 95, 31 19-3123
Geometry and Vibrations of the 1,2,3-Triazoie Anion. A Theoretical and Experimental Study C. Tornkvist,+ J. Bergman,t and B. Liedberg*,+ Loboratory of Applied Physics, Linkoping University, S-581 83 Linkoping, Sweden, and Department of Organic Chemistry, Royal Institute of Technology, S - IO0 40 Stockholm, Sweden (Received: May 10, 1990)
The equilibrium geometry and the harmonic frequencies have been calculated for the 1,2,3-triazole anion. The calculations were performed on three different levels: semiempirical (MNDO) and ab initio RHF/4-21G(N*) and MP2/6-31G*. The calculated harmonic frequencies are compared with FT-IR and FT Raman data of the 1,2,3-triazole anion in aqueous NaOH solution at pH 12. The results show that it is necessary to use a large basis set as well as to take electron correlation into account. The absolute mean deviation between experimental and calculated MP2/6-31G* frequencies was found to be 2.9%.
Introduction Triazoles and derivatives thereof are well-known as corrosion inhibitors for copper and copper alloys.I2 The two mast commonly used triazole derivatives are benzotriazole and tolyltriazole, where the active part of the molecule is the 1,2,3-triazole ring. The structure and function of adsorbed benzotriazoles on copper have been successfully studied with a large number of experimental techniques, such as ellipsometry? photoelectron spectroscopy,M surface-enhanced Raman spectroscopy (SERS),7.8 infrared reflection absorption spectroscopy (I R A S ) , ~ Yand ~ * electrochemical '~ methods." These studies have shown that nitrogens in the triazole moiety of the inhibitor molecule coordinate to copper in such a way that a three-dimensional polymeric (Cu(1)-BTA) film can grow and form a protective barrier layer on the copper surface. To understand the infrared spectra of adsorbed and solid benzotriazole, Poling made some tentative assignments of the main peaks in the spectra? Later, Rubim' presented a more accurate assignment based on Poling's work and a comparison with 1,2,3-triazole,12 1,2,4-triazoIe,l3 benzimidazole,14and imidaz01e.I~ He also introduced the concept of using the benzotriazole anion (BTA-), C,, symmetry, as a model of coordinated benzotriazole in the cupric complex (Cu(1)-BTA). This idea was later used by us6 to determine the mean orientation of benzotriazole on copper and cuprous oxide surfaces with IRAS. Orientation analysis with IRAS requires, in addition to reliable assignments, also that the direction of the transition dipole moment of each vibrational mode is known. We performed for this purpose a semiempirical MNDO calculation and a normal-coordinate analysis based on the benzotriazole anion.6 The accuracy of the triazole ring modes in that calculation is expected to be lower than for the rest of the molecule because the parameters used in the semiempirical MNDO method are not optimized for three neighboring nitrogen atom^.^^.^^ One way to overcome this problem and to gain a deeper understanding of the vibrations in the triazole part of the benzotriazole molecule is to employ extensive ab initio quantum chemical calculations of the force field. A review on that subject has been presented by Fogarasi and Pulay.18 The aim of the present study is to compare different levels of calculations of the 1,2,3-triazole anion. The equilibrium geometry and harmonic vibrations are calculated for the 1,2,3-triazole anion, Figure 1, with different basis sets and with and without electron correlation. Recently, it has been demonstrated in several papersIe2I that it is necessary to take electron correlation into account. Otherwise the coupling between different internal coordinates becomes too strong, as in conjugated and aromatic systems (cf. the calculations of the in-plane vibrations by Guo and Karplus).*O
'Linkoping University.
'Royal Institute of Technology. *To whom correspondence should be addressed.
0022-3654/91/2095-3119$02.50/0
TABLE I: Calculated Geometries and Energies 01 the 1,2,3-Triazole Anion' MNDO RHF/4-2 1G(N*) MP2/6-3 1G* R(N-N) 1.303 1.316 1.359 1.381 1.342 1.359 R(N-C) 1.412 1.379 1.387 R(C-C) 1.079 1.072 1 .ow R(C-H) 112.8 11 1.7 110.9 L(N-N-N) 107.6 106.8 106.3 L(N-N-C) 105.9 107.3 108.3 L(N-C-C) L( N-C-H) 123.1 122.6 121.5 L(C-C-H) 130.9 130.0 130.2 energy -239.734 -240.937 Bond lengths are in angstroms, angles in degrees, and energies in
hartrees.
Computational Methods The semiempirical and ab initio calculations were carried out using the GAUSSIAN 86 on a Cray XMP/48 computer. The geometries were optimized at the MNDO level,16 at the R H F (restricted Hartree-Fock) level with the base set 4-21G(N*) (1) Walker, R. Anti-Corros. Methods Muter. 1970, 17, 9. (2) Procter and Gamble, British Patent 652 339, Dec 1947. (3) Mansfeld, F.; Smith, T. Corrosion 1973, 29. 105. (4) Roberts, R. F. J. Electron Spectrosc. Relat. Phenom. 1974, 4, 273. (5) Chadwick, D.; Haskemi, T. Corros. Sci. 1978, 18, 39. (6) Nilsson, J.-0.; Tornkvist, C.; Liedberg, B. Appl. Surf. Sci. 1989, 37, 306. (7) Rubim, J.; Gutz, I. G. R.; Sala, 0.;Orville-Thomas, W. J. J . Mol. Struct. 1985, 100, 571. (8) Thierrv, D.; Levaraf. C. In Proc. 6th Eur. Svmo. Corros. Inhibitors. Ferrara; Ira& 1985, Sipt. (9) Poling, G. W. Corros. Sci. 1970, 10, 359. (IO) Tornkvist, C.; Thierry, D.; Bergman, J.; Liedberg, B.; Leygraf, C. J. Electrochem. SOC.1989, 136, 58. (11) Fox, P. G.; Lewis, G.; Boden, P. J. J . Chem. SOC.B 1979, 19, 457. (12) Borello, E.; Zecchina, A.; Guglielminotti, E. J . Chem. SOC.B 1969, 307. (13) Bougeard, D.; Le Calve, N.; Saint Roch, B.; Novak, A. J. Chem. Phys. 1976, 64, 5152. (14) Cordes, M.; Walter, J. L. Soectrochim. Acta, Part A 1968. 24. 237. (15) Cordes. M.: Walter. J. L.SDectrochim. Acta. Part A 1968.24. 1421. (16) Dewar,'M. J. S.;Ford,G. P ;McKee, M. L.; Rzepa, H. S.;Thiel, W.; Yamaguchi, Y. J . Mol. Struct. 1978, 43, 135. (17) Dewar, M. J. S.; Thiel, W. J . Am. Chem. SOC.1977, 99,4889,4907. (18) Fogarasi, G.; h l a y , P. Vibrationul Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York, 1985; Vol. 14, Chapter 3. (19) Hout, R. F.; Levi, Jr., B. A.; Hehre, W. J. J. Compur. Chem. 1982, 3, 234. (20) Guo, H.; Karplus, M. J . Chem. Phys. 1989, 91, 1719. (21) Simandiras, E. D.; Handy, N. C.; Amos, R. D. J. Phys. Chem. 1988, 92, 1739. (22) Frisch, M. J.; Binkley, J. S.;Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; Defrees, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fleuder, E. M.; Pople, J. A. GAUSSIAN 86; Carnegie-Mellon Quantum Chemistry Publishing Unit; Pittsburgh, PA, 1984. ~
~~
- .
0 1991 American Chemical Society
3120 The Journal of Physical Chemistry, Vol. 95, No. 8,1991
Figure 1. Structure and atom numbering of the 1,2,3-triazoleanion. For calculated geometries, see Table 1.
TABLE II: Definitions of Internal Coordinates for the 1,2,3-Triazole Anion' 41 = R(1.2)
= R(2J) 43 = R(1.5) 44 = R(3,4) 45 = R(4S) 46 = R(5,7) 47 = R(4,6) 48 = L ( 7 S . 1 ) - L(7,5,4) 49 46,4,3) - 46,4,5) + 4a2 + as) + b(a3 + a4) 410 = q1i = ( a - Ma2 - as) + (1 - d(a3 - a41 9i7= 7 out of (1,5,4) 413 = 6 out Of (5,4,3) q 1 4 = b ( T i + 7 5 ) + d T 2 + 74) + 7 3 415 = ( 0 - b)(~4 - T z ) + ( 1 - 0)(75 - 71) q2
" a = COS 144; b = COS 72; a i = L(1,2,3); a2 = L(5,1,2); a3 = L( 4 3 , l ) ; a4 = L(3,4,5); a5 = L(2,3,4); T~ = dhL(3,2,1,5); T Z = dhL(2,1,5,4);T ) = dhL(1,5,4,3); T4 = dhL(5,4,3,2); T5 = dhL(4,3,2,1).
(d-type polarization functions on the nitrogen atoms with an exponent of 0.8)23and finally at the MP2 level (Mdler-Plesset second-order perturbation theory to include electron correlationz4) with a 6-31G* basis set.2s The optimized geometries for the 1,2,3-triazole anion are listed in Table I. The optimized geometries were later used as equilibrium geometries in the calculations of the harmonic force fields (second derivatives of the energy with respect to coordinates) and transition dipole moments. GAUSSIAN 86 calculates the force field analytically in the RHF method. However, in the MNDO and MP2 methods, the force fields were obtained by numerical differentiation of the gradient of displaced Cartesian coordinates from the equilibrium geometry. This was a minor problem in the MNDO calculation, but the MP2 calculation was lengthy, even on a Cray XMP/48. The obtained force fields and transition dipole moments together with the geometries were transferred to a separate program written by us for postprocessing of vibrational data. In the postprocessing program the Cartesian force field and Cartesian transition dipole moments are transformed into internal coordinates as described in ref 18. The transformation into internal coordinates gives a chemical picture of the vibrational modes. The harmonic vibrations were determined according to the G F matrix method by Wilson et a1.26 The internal coordinates, see Table 11, were chosen as suggested by Fogarasi and Pulay.I* Experimental Section 1,2,3-Triazole can be prepared by a number of literature procedures. The material used in this study was prepared according to l d d ~ n . ~ ' * ~This * method involves reaction of 4-
Tornkvist et al. methoxybenzyl azide with acetylene-dicarboxylic acid, pyrolysis of the resulting 4,5-dicarboxylic acid, and finally deprotection of the resulting 1 -(4-methoxybenzyl)-l,2,3-triazoleby heating it in trifluoroacetic acid in the presence of anisole. Another method given by B e g t r ~ p ~ ~involving -~O reaction of HN3 with propiolic acid followed by thermal decarboxylation was also studied, and it was concluded that this method could be used for small-scale preparations of 1,2,3-triazole. Infrared and Raman spectra of the 1,2,3-triazole anion were recorded in aqueous 0.5 M NaOH solution at pH 12. The infrared spectrum of the solution was recorded on a Bruker IFS 113v FTIR spectrometer. The liquid cell had a length of 7 pm, and the window material was CaF2. The lower cutoff around 950 cm-I was due to the window material, and the upper cutoff around 1500 cm-I was due to strong water absorption. Infrared diffuse reflectance spectroscopy was used to study the 1,2,3-triazole anion adsorbed on KBr powder in the 1000-500-cm-' region. The Raman spectrum was recorded on a Bruker IFS 66 unit with a FRA 106 accessory, and a Nd:YAG laser was used as excitation source (A = 1.064 pm).
Results and Discussion The optimized geometries of the 1,2,3-triazole anion are shown in Table I. The differences between the three different methods show a maximum deviation in bond length of 0.025 A, except for the N-N bond, where it is found to be 0.056 A. MP2 calculations are known to give a good geometry, although the bond lengths generally are overestimated a few thousandths of an angstrom for a single bond and slightly more for a double bond.*I However, the error in bond length is expected to be lower in the MP2/631G* calculation as compared to the MNDO and RHF/4-21G(N*) calculations. On the other hand, MP2/6-31G* calculations are very time consuming, more expensive, and limited, in practice, to smaller molecules. The only quantum chemical calculation of the equilibrium geometry of the 1,2,3-triazole anion is, to our knowledge, the work reported by Tanaka et al.3' They performed CNDO calculations on nonoptimized geometries of different 1,2,3-triazole derivatives in order to understand addition reactions and used the following bond lengths: R(N-N) = 1.375 A, R(C-N) = 1.34 A, R(C-C) = 1.44 A. Their bond lengths are similar to those obtained at the MP2/6-31G* level, except for the C-C bond length. The equilibrium geometry for the benzotriazole anion has been calculated by using RHF/6-31G.j2 The following bond lengths of the triazole moiety were obtained in that case: R(N-N) = 1.327 A, R(C-N) = 1.362 A, R(C-C) 1.406 A. These bond lengths agree with the RHF/4-21G(N*) values in Table I. The minor differences are most likely attributed to the influence of the benzene ring and the larger basis set. The calculated harmonic vibrations, for the three different levels of approximation, are shown in Table 111 together with experimental FT-IR and FT Raman data. The force field of the most extensive calculation (MP2/6-3 1G*) is transformed to internal coordinates and shown in Table IV. A full description of the force field from the calculations and other data are available upon request from the authors. The best fit to the experimental lT-IR and FT Raman data is obtained with the (MP2/6-31G*) method, which gives an absolute mean deviation of 2.9% between experimental and calculated frequencies. The vibrational frequencies obtained from the medium-sized basis set RHF/4-21G(N*) were not of the same quality as the calculations of similar heterocyclic five-membered ring systems, such as pyrrole33 and imidazole.34 Simple scaling (scaled quantum mechanics, SQM) of the ~
Pongor. G.; Fogarasi, G. J . Mol. Specrrosc. 1985, 114, 445. M0ller. C.; Plesset, M. S. Phys. Reu. 1934. 46, 618. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations;Dover Publications: New York, 1980. (27) Iddon, B. Chemistry of Heterocyclic Compounds; Kovac, J., Zalupsky, P., Eds.; Elsevier: Amsterdam, 1988; pp 24-45. (28) Iddon, B. Personal communication. (23) (24) (25) (26)
(29) Begtrup, M.;Nielsen, C. J.; Nygaard, L.; Samdal, S.; Sjsgren, C. E.; Ssrensen, G. 0. Acta. Chem. Scand. A 1988, 42, 500. (30) Begtrup, M.Personal communication. (31) Tanaka, Y.; Miller, S. I. Tetruhedron 1973, 29, 3285. (32) Tomis, F.; Abboud, J.-L. M.; Laynez, L.; Notario, R.; Santos, L.; Nilsson, S.0.; Catalin, J.; Claramunt, R. M.;Elguero, J. J . Am. Chem. Soc. 1989, 1 1 1 , 7348. (33) Xie, Y . ; Fan, K.; Boggs, J. E. Mol. Phys. 1986, 58, 401. (34) Fan, K.; Xie, Y . ;Boggs, J. E. J . Mol. Srrucr. 1986, 136, 339.
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3121
The 1,2,3-Triazole Anion
TABLE 111: Harmonic Vibrational Frequencies a d IR Intensities (in Parentheses. km/mol) for the 1.2.3-Triazole Anion4 we, cm-l v , cm-l MNDO RHF 4-21G(N*) MP2 6-31G* FT-IR FT Raman A% MP2/6-31G* A, Modes 1 3497 (0.8) 3355 (67.4) 3257 (55.7) 3135 s +3.9 2 1657 (6.7) 1576 (0.6) 1523 (0.2) 1439 m 1439 s +5.8 3 1518 (1.4) 1301 (10.4) 1274 (0.002) 1207 vs +5.6 4 1468 (0.9) 1300 (0.06) 1139 (0.3) 1146 s 1 I43 vs -0.4 5 1179 (0.3) 1193 (1.6) 1115 (8.9) 1097 m 1096 m +1.7 6 979 (0.02) 1080 (1.9) 944 (1.7) 963 s 964 m -0.2
B, Modes 7 8 9
IO 11
3484 (0.7) 1656 (16.9) 1517 (1.0) 1219 (1.9) 921 (4.2)
3316 (61.5) 1578 (7.3) 1241 (0.4) 1214 (26.2) 1075 (14.1)
3231 (40.1) 1442 (2.9) 1217 (0.3) 1077 (31.6) 961 (13.1)
3116 s 1412 vw
1412 m 1230 w 1068 vs
12 13
919 (0) 581 (0)
948 (0) 756 (0)
A, Modes 726 (0) 628 (0)
14 15
875 (28.4) 674 ( 1 . 1 )
886 (29.9) 819 (0.08)
759 (28.3) 724 (3.1)
+3.7 +2.1 -1.1 +0.8 -0.1
972 m
+o. 1
725 vw 668 vw
-6.0
B2 Modes -5.1 -6.3
800 mb 773 mb
"he calculations are compared with experimental frequencies from FT-IR and FT Raman data of the 1,2,3-triazole anion in 0.5 M NaOH solution at pH 12. Note that the assignments to experimental data are based on the MP2 calculation. bThese bands are obtained from a F N R diffuse reflectance measurement of absorbed and dried 1,2,3-triazole in KBr powder. TABLE IV: Force Field in Internal Coordinates (Definitions in Table 11) for internal coord I 2 3 4 5 6 7 I 5.667 2 0.544 5.667 3 0.363 0.261 7.141 4 0.261 0.363 -0.031 7.141 5 -0.055 -0.055 0.501 0.501 7.648 6 -0.008 -0.098 0.132 0.034 0.054 5.764 7 -0.098 -0.008 0.034 0.132 0.054 0.012 5.764 8 -0.033 -0.017 0.190 0.024 -0.096 -0.016 -0.007 9 -0.017 -0.033 0.024 0.190 -0.096 -0.007 -0.016 IO -0.002 -0.002 -0.571 -0.571 0.635 -0.068 -0.068 11 -0.696 0.696 0.420 -0.420 0.000 -0.172 0.172 12 13 14 15
the 1,2,3-Triazole Anion4 8
9
0.470 -0.005
0.470
IO
11
12
13
0.337 -0.001 -0.214 0.072
0.337 0.214 0.072
14
15
-0.100 -0.100 -0.012
2.192 0.012 0.000
1.8888
0.549 0.000 0.426
"The data are obtained from the MP2/6-31GS calculation. The units of F,j are in mdyn/A, mdyn/rad, or mdyn A/rad2 depending on the units
of q, and q,,
RHF/4-21G(N*) force field using transferable scaling constants, as suggested by Pulay et a1.,I8 did not improve the results in this particular case. Table V shows the diagonal elements of the three different force fields. The most pronounced differences between the three computational methods occur, as expected, in the calculations of the N-N stretching force constants. The MP2/6-3 IG* calculation gives force constants F,,land F2,2, which are respectively 50 and 75% of those obtained with the MNDO and RHF/4-2lG(N*) methods, whereas the stretching force constants of the C-C, C-N, and C-H bonds generally show much smaller deviations between the computational methods. The large changes in the N-N stretching force constants as well as the N-N bond lengths are assumed to be almost entirely due to electron correlation. This statement is based on a comparison between the present RHF/ 4-21G(N*) calculation of the 1,2,3-triazole anion and the RHF/6-3 IG calculation of the benzotriazole which both give comparable N-N bond lengths. Bougeard et al.13 have empirically estimated the force field of 1H-l,2,4-triazole and calculated the N-N stretching force constant to be 6.20 mdyn/A. This force constant is from a single bond in 1H- 1,2,4-triazole and appears in comparison with the present MP2/6-31G* value of the 1,2,3-triazole anion to be slightly too large. The C-N stretching force constant and bond length are almost the same in the RHF/4-21 (N*) and MP2/6-31G* calculations.
TABLE V Diagonal Elements of the Force Fields from MNDO, RHF/4-2lG(N*), and MP2/6-31G* Calculations (See Table IV for Units) MNDO RHF/4-21 G(N*) MP2/6-3 lG*
FI.1 F2.2 F3,3 F4.4
F5S F6.6
F7.7 F8,8
F9.9 Fl0,lO
FI1.11 F12.12 F14.14
F15.15
1 1.648 1 1.648 9.444 9.444 8.774 6.739 6.739 0.48 1 0.481 2.024 1.836 0.426
7.593 7.593 7.869 7.869 7.940 6.092 6.092 0.547 0.547 2.588 2.323 0.488
0.426
0.488
0.408 0.3 19
0.677 0.492
5.667 5.667 7.141 7.141 7.648 5.764 5.764 0.470 0.470 2.192 1.888 0.337 0.337 0.549 0.426
A related calculation of imidazole at the RHF/4-21G levelMgave the following C-N stretching force constants: 7.22,7.04, and 8.90 mdyn/A (6.58, 6.42, and 8.11 mdyn/A, scaled according to SQM). The first two force constants are from single bonds and the last one from a double bond. A calculation of pyrrole at the RHF/4-21G level33gave a C-N stretching force constant of 7.23
3122 The Journal of Physical Chemistry, Vol. 95, No. 8,1991
Tornkvist et al.
TABLE VI: MP2/6-31C* Calculated Wavenumbers and Potential Energy Distribution (PED, percent) among Internal Coordinates MP2 6-3 I G* u.. cm-I PED.% aDorox description ~~
~~~
3257 1523 1274 1 I39 1115 944
C-H str C-C str C-N str C C str + C-H def N-N str + ring def ring def
II
323 I 1442 1217 1077 96 1
C-H C-N str + C-H def N-N + C-N str N-N str + C-H def ring def
12 13
726 628
C-H wagging ring torsion
14 15
159 724
C-H wagging ring torsion
1
2 3 4 5 6 7
8 9
IO
"The sign is from the corresponding L matrix element. mdyn/A (or 6.34 mdyn/A scaled according to SQM). The C-N bond in pyrrole is a single bond, and its force constant is comparable with the single bond in imidazole. Simandiras et aL2' have calculated the harmonic vibrations at the MP2/DZP level for pyrrole. They did not transform the force field into internal coordinates, which unfortunately makes a direct comparison with the present data very difficult. In lH-1,2,4-triazole the C-N stretching force constants were found to be 6.20, 6.50, 7.10, and 7.10 mdyn/A," respectively, where the first two are from single bonds and the last two from double bonds. The value of the C-N force constant, 7.141 mdyn/A, from our MP2/6-31GS calculation suggests, in accordance to the discussions above, that the C-N bond in the 1,2,3-triazole anion is between a single bond and a double bond. Bending motions and especially out-of-plane bending modes are expected to be less accurately reproduced, even at the MP2/6-31G* level. Simandiras et al.35 have reported that it is necessary to use a triple-!: basis set plus two sets of polarization functions and one set o f f functions (TZ2P+f) and inclusion of electron correlation in order to be able to calculate the out-of-plane bending r vibration in ethyne within 1%. Tables I11 and VI show that the dP2/6-31G* calculation gives AI and B, in-plane bending frequencies in good agreement with the experimental data, whereas most of the A2 and B2 out-of-plane bending modes are less accurately reproduced. The results obtained for the A, and B1 modes are in fact surprisingly good. If the bending modes were calculated in the harmonic limit, one would expect a greater positive difference compared to experimental data, perhaps a few percent,35 which then could be addressed to anharmonicity. However, to do harmonic limit calculations requires very large basis sets, and the quantum chemical calculations are very time consuming and behind the scope of this paper. The assignments given in Table I11 are based on the MP2/631G* calculation, However, there are still a few uncertainties about the exact assignments. The band at 1230 cm-' in the IR and the band 1207 cm-l in the Raman spectra, see Figure 2, are assigned to the calculated modes at 1217 (B,) and 1274 cm-I (Al), respectively. The reason for switching these two modes is that the calculated IR intensity of the BI mode is much higher than that of the A, mode, and that AI modes generally are more intense than B, modes in Raman spectroscopy. The next pair of uncertainly assigned modes are the singlet at 963 cm-l in the IR and the doublet at 964 and 972 cm-l in the Raman spectra. They are also switched according to the intensity data frcm the MP2/6-31G* calculation. In both these cases a larger error in (35) Simandiras, E.: Rice, J. E.; Lee, T. J.; Amos, R. D.;Handy, N. C. J . Chem. Phys. 1968.88, 3187.
1500
1300
1100 900 Wavenumber cm-1
700
500
Figure 2. FT-IR (A) and FT Raman (e) spectra of the 1,2,3-triazole anion in 0.5 M NaOH at pH 12. The FT-IR spectrum of the 1,2,3triazole anion in aqueous NaOH was possible to record only in the interval 1600-950 cm-'. The upper limit is due to water absorption, and the lower limit is due to the CaF2 windows. The low-frequency FT-IR spectrum (C) (1000-500 cm-l) was taken on adsorbed and dried 1,2,3triazole in KBr powder by using diffuse reflectance spectroscopy.
frequencies is obtained compared to if the modes were not switched. The IR and Raman spectra of benzotriazole (BTAH) and benzotriazole anion and its Cu(1)-BTA complex have been discussed earlier by Rubim et al.' and the assignments were based on spectral data from different solutions, adsorbed species, and analogues derivatives. Nilsson et aL6 calculated the force field and vibrational frequencies by using the MNDO method and compared their results with IR spectra of BTA- and the Cu(I)-BTA complex. The characteristic peaks in the IR spectrum of the triazole moiety in BTA- occur at 1163, 1 134, and 11 15 m-', respectively, and a broad band with a main peak a t 1151 cm-l occurs in the spectrum of the Cu(1)-BTA complex. The spectrum of the 1,2,3-triazole anion exhibits three bands in that spectral region at 1143, 1096, and 1068 cm-I, which are assigned to the calculated modes at 1139 (Al), 11 15 (Al), and 1077 cm-I (Bl), respectively, Table 111. The modes at 1115 and 1077 cm-' are according to the potential energy distribution (PED) in Table VI essentially due to N-N stretching and ring modes, whereas the 1139-cm-l peak also includes contributions from C-H bending and C-C stretching modes. This result supports the proposal by
3123
J . Phys. Chem. 1991, 95, 3123-3128 Rubim’ that the Raman band at 1165 cm-’ for the BTA- molecule should be associated with the N-N-N stretching and not to C-H in-plane deformations later proposed by us! Our (scaled) MNDO results6 indicated that the N-N stretching modes of BTA- should occur at much higher frequencies, -1500 cm-’. The same trend is obtained in the present study of the 1,2,3-triazole anion, where both the MNDO and the RHF/4-21G(N*) methods overestimte the N-N stretching force constants, Table V, and thereby also the vibrational frequencies, Table 111. This observation furthermore supports our previous proposal that electron correlation is absolutely necessary if one intends to calculate the force field for three neighboring nitrogens, with a reasonable accuracy. A similar paper on 1H-and 2H-1,2,3-triazole follows this article.36
Conclusions in this paper we have calculated the equilibrium geometry and harmonic frequencies of the 1,2,3-triazole anion at three different quantum chemical levels. A comparison with experimental FT-IR and FT Raman data reveals that it is necessary to use a large basis set with a high degree of flexibility and electron correlation in the quantum chemical model, in order to reproduce the geometry (36) TBrnkvist, C.; Bergman, J.; Liedberg, B. J . Phys. Chem., following paper in this issue.
and the vibrational frequencies of the 1,2,3-triazole anion. The most sophisticated MP2/6-3 lG* calculation shows good agreement with experimental data and gives an absolute mean deviation in vibrational frequencies of 2.9%. The most significant difference between the quantum chemical methods MNDO, RHF/4-21G(N*), and MP2/6-31GS appears in the calculation of the N-N stretching force constant. The MNDO method, in particular, gives a substantial overestimation of the N-N stretching force constant and N-N stretching modes, which are calculated to be ~ 3 5 cm-’ 0 too high in frequency. This upward shift is comparable to that achieved for the benzotriazole molecule.6 The present approach of introducing electron correlation in the quantum chemical method gives a much better fit to experimental data and provides a good basis for further studies of heterocyclic compounds.
Acknowledgment. This work was supported by a grant from the Engineering Research Council of STU (STUF). The supercomputer calculations were performed by grants from Swedish Natural Science Research Council (NFR) and National Supercomputer Center at Linkoping University. We also thank Dr. J. Sawatzki and Dr. J. Gast at Bruker FRG for the FT Raman measurements. Registry No. 1,2,3-Triazoleanion, 5 17 19-91-6.
Correlated ab Initio Geometries and Vibrations of 1H- and 2H-1,2,3-Triazole C. Tornkvist; J. Bergman,$ and B. Liedberg*” Laboratory of Applied Physics, Linkoping University, S-581 83 Linkoping, Sweden, and Department of Organic Chemistry, Royal Institute of Technology, S - 100 40 Stockholm, Sweden (Received: May 10, 1990)
The equilibrium geometry and the harmonic force field of 1 H- and 2H-1,2,3-triazolehave been calculated at the second-order Mdler-Plesset level (MP2) with the 6-3 1G* basis set. The obtained rotational constants from the equilibrium geometries agree very well with experimental data of several isotopic species. We believe therefore that the present geometries are of very high quality and that the calculated force fields and harmonic vibrational frequencies can provide a good basis for a future high-resolution infrared study of 1,2,3-triazolesand analogous molecules. The possibility of using the present calculation in the study of the tautomerism of 1,2,3-triazoles is also discussed.
Introduction 1,2,3-Triazoles are important in many applications such as light stabilizers, optical brightening agents, and corrosion inhibitors.Is2 Several studies have paid attention to the tautomerism between 1H- and 2H-1,2,3-triazole. Borello et al. claimed that the 1H1,2,3-triazole was the dominant tautomer from gas-phase IR ~pectroscopy.~The spectrum showed five C-type bands in the spectral region 900-500 cm-I, which is expected for the 1H form (C, symmetry), Figure 1. The 2 H form, Figure 1, on the other hand, has only three IR-active modes in that region and was, according to its symmetry, disregarded as the most abundant tautomer. Stiefvater et al.’ argued from microwave spectroscopy experiments that the IH-l,2,3-triazole was the predominant tautomer in the gas phase. However, Sarensen et aL5 pointed out that this was not a conclusive proof since the assignment was based only on an a-type transition, but both a- and b-type transitions are symmetry allowed for the 1H form in double-resonance microwave spectroscopy. In their microwave study of N-deuterio1,2,3-triazole the results suggested that the 1-deuterio form was the dominant tautomer in the s ~ e c t r u m . ~However. thev did not exclude the wssibilitv that the’2-deuterio form could bedmesent. even in comparable -amounts, because of the vanishini dipole
’*Royal Link6ping University. Institute of Technology.
*Towhom correspondence should be addressed.
0022-3654/91/2095-3 123$02.50/0
moment of this tautomer. Recently Begtrup et aL6 presented a paper on a combined study of microwave spectroscopy, gas-phase electron diffraction, and a b initio calculations at the R H F (restricted Hartree-Fock) level of the parent molecule, 15N3and their N deuterated species. The results showed that the molecule exists in two planar tautomer forms: 1H with C, symmetry and 2 H with C, symmetry. The estimated abundance was 1H.2H 1:lOOO at room temperature, based on microwave intensity measurements. Albert and Taylor’ recently discussed the influence of the solvent on the equilibrium of the two tautomers in terms of lone-pair repulsion. Catalln et aI.* studied the tautomerism of 1,2,3-triazole and 3(5)-methylpyrazole and their cations both quantum chemically (ab initio) and experimentally. Recently9 they extended
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(1) Gilchrist, T. L.; Gymer, G. E. Advances in Heterocyclic Chemistry; Academic Press: New York, 1974; pp 33-85. (2) TBrnkvist, C.; Thierry, D.; Bergman, J.; Liedberg, B.; Leygraf, C. J . Electrochem. SOC.1989, 136, 58. (3) Borello, E.; Zecchina, A.; Guglielminotti,E. J. Chem. Soc. B 1969, 307. (4) Stiefvater, 0.L.; Jones, H.; Sheridan, J. Spectrochim. Acta 1970,26A, 825.
(5) Ssrensen, G. 0.;Nygaard, L.; Begtrup, M. J . Chem. SM., Chem. Commun. 1974i 605. ( 6 ) Begtrup, M.; Nielsen, C. J.; Nygaard, L.; Samdal, S.;Sjagren, C. E.; Sarensen, G. 0. Acta Chem. Scand. A 1988, 42, 500. (7) Albert, A.; Taylor, P. J. J . Chem. Soc., Perkin Trans. 2 1989, 1903. (8) Catalin, J.; S6nchez-Cabezudo. M.; de Paz, J. L. G.; Elguero, J.; Taft, R. W.; Anvia, F. J . Comput. Chem. 1989, 10, 426.
0 1991 American Chemical Society
