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Structure and High-Resolution IR Spectroscopy of 1,2,4-Triazine Vapor
JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.
192, 331–337 (1998)
MS987707
Structure and High-Resolution IR Spectroscopy of 1,2,4-Triazine Vapor Michael H. Palmer,*,1 Robert R. J. Maier,* Flemming Hegelund,† and David A. Newnham‡ *Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, UK; †Department of Chemistry, Aarhus University, DK-8000, Aarhus-C, Denmark; and ‡Space Science Department, Rutherford Appleton Laboratory, Chilton, Oxford OX11 0QX, England, UK Received May 14, 1998; in revised form June 29, 1998
The high-resolution infrared bands centered at 769 and 1043 cm21 for 1,2,4-triazine C3H3N3 have been analyzed, using the full asymmetric rotor Hamilton of Watson in A-reduced form using IIIr -representation. All the ground state rotation constants and three of the quartic centrifugal distortion constants could be determined with high precision from a simultaneous analysis of the two bands. The standard deviation of fit was 0.00077 cm21. The upper state constants for the two bands have been determined with similar precision. The ground state rotational constants obtained from the present analysis are very similar to those predicted from the ab initio study of the equilibrium structure. This strongly suggests that the similarity in several of the ring bond lengths, predicted in the latter, is indeed real. © 1998 Academic Press 1. INTRODUCTION
1,2,4-Triazine(I) is one of the least studied of the heteroaromatic (azine) series C6 –nH6 –nNn, where the known compounds have n 5 0–4. This is because the ring system is readily hydrolyzed, particularly in the presence of moisture and acids (1, 2). Recently, we reported a wide-ranging study of the excited states of I, by combined theoretical and experimental (UV 1 VUV and electron impact spectroscopy) methods (3). The equilibrium structure formed a basis for the theoretical calculations. There is no structural information available for this molecule. We have been unable to obtain a single crystal x-ray diffraction structure, owing to crystal twinning. The three 14N centers and asymmetric rotor structure make microwave spectral determination complex. The theoretical structures obtained by SCF or MP2 methods with a variety of basis sets consistently obtained all ring bonds except the C5C6 bond near 1.33 Å. This makes an electron diffraction study somewhat problematic. The purpose of the present investigation, is to present an analysis of two bands in the high-resolution infrared spectrum, one centered at 1043 cm21 and one at 769 cm21, and to compare the ground state rotational constants with theoretical data. 2. THEORETICAL STRUCTURE AND FORCE FIELD
1,2,4-Triazine, C3H3N3 is thought to be planar, as are the other known azines. A series of basis sets were used in the previous investigation (3), to determine the theoretical equilibrium structure, assuming a planar molecule with C s symmetry. The largest ab initio basis set was triple-zeta in the valence 1
To whom correspondence should be addressed (E-mail: m.h.palmer@ ed.ac.uk).
shell with polarization functions and included Moller–Plesset correlation to second order (TZVP 1 MP2). The final structure, used here, shows 4 ring bonds in the range 1.330 –1.350 Å, with only the C5–C6 bond differing at 1.394 Å (see Fig. 1). The dipole moment lies very close to the N2–C5H axis, which in turn lies at 44.5° ( um a ) to the a axis; thus m a and m b are nearly equal (1.957 and 1.932 D, respectively). The equilibrium structure of the molecule is thus a moderate oblate asymmetric rotor, with an asymmetry parameter k 5 0.72. The harmonic force field predictions of vibration frequencies, expressed in wavenumbers, and integrated absorption intensities are given in Table 1; these are determined by analytic derivatives at the equilibrium geometry of the TZVP 1 MP2 determined structure. In the present work, we have tried to assign the fundamentals of 1,2,4-triazine from the infrared and Raman survey spectra, on the basis of the predictions in Table 1. However, to date, we have not been able to obtain all of the necessary IR data at low frequencies; thus we have not been able to establish an unambiguous assignment, although alignment of the two lists of experimental and theoretical data is one possibility. Overall, we have therefore decided not to include a comparison of the theoretical and experimental data in the Table. In Table 1, the molecule lies in the center-of-charge system (x, y plane), where the y axis is effectively identical to the C3PC6 direction. Fifteen of the normal vibrations, n1–n15, are in-plane, being totally symmetric ( A9). The remaining six, n16–n21, are nonplanar corresponding to A0 symmetry. The totally symmetric vibrations give rise to a,b-hybrid bands, for which the strongest transitions are governed by the symmetric top rotational selection rule DKc 5 61. The vibrations of A0 symmetry yield pure c-type bands whose strongest transitions are characterized by the selection rule DKc 5 0.
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TABLE 1 Calculated Harmonic Frequencies and Integrated Absorption Intensities
FIG. 1. Equilibrium structure for 1,2,4-triazine showing bond lengths and angles in the inertial axis frame.
3. EXPERIMENTAL SECTION
The compound, prepared by previously reported procedures (1, 4), had spectra very similar in appearance to those of previous studies (3), and in particular, the infrared spectrum in the gas phase shows bands which are close to those reported for the liquid film (1, 4). Laboratory measurements of the midinfrared absorption spectra of 1,2,4-triazine vapor were made using a Bruker IFS 120HR high-resolution Fourier transform spectrometer (FTS), in the Molecular Spectroscopy Facility at the Rutherford Appleton Laboratory. It used a globar broadband IR source, KBr/Ge beam splitter, and liquid nitrogen– cooled mercury cadmium telluride (MCT) detectors. The FTS instrument resolution (R) is defined as R 5 0.9/(MOPD), where MOPD is the maximum optical path difference, which corresponds to the full width at half-maximum (FWHM) with the use of triangular apodization. Sampling fold points and optical and electronic filters were selected to eliminate aliasing and reduce detector nonlinearity effects which would otherwise lead to errors in the recorded spectra. The band-pass optical filters were mounted close to a focus point in the optical beam between the interferometer and gas cell and tilted to reduce optical “channeling” interference effects in the recorded spectra. Throughout the measurements all optical paths inside the FTS were evacuated by a turbomolecular vacuum pump to pressures lower than 1024 hPa, reducing absorption by atmospheric gases in the optical path to negligible levels.
The synthesized sample of 1,2,4-triazine was purified by vacuum distillation and the vapor transferred to a 26.1-cm optical path length stainless steel gas cell via clean Pyrex glass/PTFE vacuum lines. Sample pressures were measured using a calibrated 10-Torr Baratron capacitance gauge (MKS Type 390). The gas cell was fitted with KBr optical windows and thermistor temperature sensors which were attached in thermal contact with the exterior of the cell. Spectra were recorded under the measurement conditions listed in Table 2. Recorded interferograms were transformed using Mertz phase correction and weak Norton–Beer apodization. Under the measurement conditions of each sample measurement, background spectra were recorded at lower spectral resolution with the gas cell evacuated, interpolated onto a wavenumber grid matching that of the sample spectra, and ratioed with sample spectra to give transmittance spectra. The absolute wavenumber scale of the spectra was that determined from a calibration of the internal FTS HeNe laser. The accuracy of this calibration was checked by comparing
Copyright © 1998 by Academic Press
TABLE 2 Instrument Settings
HIGH-RESOLUTION IR SPECTROSCOPY OF 1,2,4-TRIAZINE
333
FIG. 2. A survey spectrum of the n12 (or n13) band of 1,2,4-triazine around 1043 cm21. The central Q branch of the strong a-type band component of n12 is observed at 1043.5 cm21.
line-position measurements of standard calibration gases with literature values. The center positions of the absorption lines were obtained from the raw data, by use of the ORIGIN software. Raw spectra were smoothed three to five times in succession using a Savitzky–Golay five point smoothing routine before applying the peak–find function of ORIGIN. The FWHM and peak position of several peaks were monitored after each smoothing operation, and smoothing was applied just until peak broadening started for prominent peaks. For minor peaks a small amount of peak broadening of up to 0.0065 cm21 was permitted, to facilitate a more accurate and efficient run of the peak–find routine. The effect of the smoothing routine on the peak position was a fraction of the FWHM and therefore negligible. The peak–finding was gradually refined to include increasingly minor features until typically 80 –100 peak positions per wavenumber around the 769- and 1043-cm21 bands were identified. Features located on the shoulders of prominent peaks were not recognized automatically and have been par-
tially excluded, as they might lead to a potentially strong shift in peak position. The survey scans of the two bands studied in the present work are shown in Figs. 2 and 3. 4. ASSIGNMENT OF SPECTRA
The harmonic force field predictions suggest that the 1043cm21 band may be assigned either as n 12 ( A9) or n 13 ( A9). For both fundamental bands, the a-type component is predicted to be ca. three times stronger than the b-type component. Since no strong central Q branch is expected for a b-type component, this prediction is in good accordance with our observation of such a Q branch in the band. However, at present we are not able to choose between the two possible assignments above for the 1043-cm21 band, and we arbitrarily assign it as n 12 ( A9). The n12 band appears as a typical a-type band of a near oblate planar asymmetric top molecule. Intense P P- and R Rbranch clusters spaced approximately ( A 1 B)/ 2 ; 0.21 cm21, dominate the spectrum. Each cluster consists of a series
FIG. 3. A survey spectrum of the n18 (or n19) band of 1,2,4-triaziine around 769 cm21. The Q branch of the strong c-type band n18 is observed at 768.75 cm21.
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FIG. 4. Part of the P P branch of the n12 (or n13) band of 1,2,4-triazine at 1043 cm21. The assignment of P P-cluster structure for 2J0 2 K 0c 5 21 2 24 is shown. Unresolved R Q branches with K 0c 5 20 2 23 are also indicated. The upper part of the figure shows a stick simulation of the region using the constants from Table 3.
of lines for which 2J0 2 K 0c ) is a constant value. Each individual line in a cluster may be characterized by ( J0, K 0c ) 5 ( J 2 t, J 2 2t), where t 5 0, 1, 2, 3, . . . , J/ 2, or ( J 2 1)/ 2 for J even or odd, respectively. The strongest line within each cluster has t 5 0; i.e., it is R R J ( J) or P P J ( J), and the intensity decreases with increasing t. For high t, lines are split due to the molecular asymmetry, and they become difficult to observe in the spectrum due to their low intensity. The assignment was initiated using the method of ground state combination differences (GSCDs) based on the rotational constants calculated from the present prediction of the equilibrium structure. The rotational ground state constants were iteratively improved during the assignment process. The assignment was facilitated by making use of computer assisted Loomis–Wood plot type diagrams, where ordering of the transitions according to upper state energies, to take into account GSCDs, was originally proposed by Nakagawa and Overend (5). In Figs. 4 and 5, the assignments of parts of the P P and R R branches are shown. In the P P branch, a series of sharp unresolved R Q branches appear. The assignment to K c of some of them is also indicated in Fig. 4. Correspondingly, P Q branches are observed in the R R-branch region. However, in this case the
Q branches are broader, and they are not so obvious in the spectrum, as may be seen from Fig. 5. In the band center region near 1043.5 cm21, the molecular asymmetry gives rise to an intense Q-branch structure consisting of P Q and R Q branches for low K c . This is a characteristic feature of a-type bands of near oblate asymmetric tops. However, the lines in this region are so congested, that no assignments of individual transitions are possible. Our final assignment of the n 12 ( A9) band comprises P P- and R R-branch transitions up to J 5 70 and K c 5 66. Asymmetry splitting is observed for K c , 8. The asymmetry split lines are weak, and only short series of such lines are observed. The split lines follow a-type band selection rules; i.e., DK a is even. We also searched for asymmetry split lines in the b component of n12, i.e., lines with DK a odd, but they were too weak to be identified with certainty. The survey spectrum of the 769-cm21 band is shown in Fig. 3. Two Q branches appear near the band center; a strong Q branch at 768.75 cm21, and a weaker one at 770.05 cm21. From our harmonic force field prediction, three fundamental bands may appear in this region: n 14 ( A9), n 18 ( A0), and n 19 ( A0). The P and R branches belonging to the strong Q branch show cluster structure with groups of lines spaced ca.
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HIGH-RESOLUTION IR SPECTROSCOPY OF 1,2,4-TRIAZINE
335
FIG. 5. Part of the R R branch of the n12 (or n13) band of 1,2,4-triazine at 1043 cm21. The assignment of R R-cluster structure for 2J0 2 K 0c 5 16 2 20 is shown. The upper part of the figure shows a stick simulation of the region using the constants from Table 3. The P Q branches which are widespread are marked on the simulation. They are only weakly discernable in the observed spectrum.
( A 1 B) ; 0.42 cm21. This indicates that the band is of c type; the observed cluster structure arises from more or less resolved K c structure of individual Q P- and Q R-branch lines, each characterized by J0. At present we are not able to decide whether the 769-cm21 c-type band is n18 or n19, and we arbitrarily assign it as n 18 ( A0). No rotational P- and R-branch structure belonging to the weaker Q branch at 770.05 cm21 has been observed. We are therefore not able to assign this Q branch to a fundamental vibration with certainty. However, our force field predictions suggest that it might be due to n 14 ( A9). A cluster in the n18 band consists of the individual K c components of the Q P- or Q R-branch lines for which J0 is a constant value. The strongest lines within each cluster have K 0c 5 0, and the intensity diminishes slowly with K 0c increasing. This is quite different from the situation for the a-type band clusters discussed above, where the strongest line was for K 0c 5 J0. Furthermore, for low K c there is systematic overlap of lines because energy levels J, K a , K c and J, K a , K c 1 1 with K a 5 J 2 K c become systematically degenerate for high J and low K c . This means, e.g., that the line series Q R K ( J) and Q R K11 ( J) with K a 5 J 2 K overlap (K 5 K c ). Furthermore, the lines in each cluster converge to a bandhead for high K c .
This bandhead formation is particularly prominent in the Q Rbranch region. For the Q P branch, the bandheads are clearly observable too, although they are somewhat broader. In Fig. 6, a small part of the assignment of the Q R branch of n18 is shown. In the central Q Q branch no assignments are possible because of the high degree of blending of lines. Our final assignment of the n18 band includes Q P- and Q R-branch lines up to J 5 70 and K c 5 33. Asymmetry splitting is observed for K c , 13. In contrast to the a-type band n12, the c-type band n18 shows long series of asymmetry split lines of moderate intensity. 5. GROUND AND UPPER STATE SPECTROSCOPIC CONSTANTS
To our knowledge, no previous microwave or high-resolution infrared study has been published on the ground state of 1,2,4-triazine. Precise ground state rotational and centrifugal distortion constants of this molecule have therefore not been determined before by spectroscopic methods. Our assignments of the rotational structure of the n12 and n18 bands of 1,2,4-triazine yield a number of ground state combi-
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FIG. 6. Part of the Q R branch of the c-type band n18 (or n19) of 1,2,4-triazine around 1043 cm21. The assignments to K c of the fine structure of the Q R(29) and part of the Q R(28) branches are shown. The value of the K a quantum number is given as a parity. For plus parity, K a 5 J 2 K c ; for negative parity, K a 5 J 2 K c 1 1. For low K c , the components are pairwise coincident. See text. For Q R(28), splitting is observed for the pair K c 5 5 2 , 4 1 . For K c . 12, lines are not resolved, and a stick diagram of Q R(29), using the constants of Table 3, shows a prediction of K c splitting and bandhead formation.
nation differences (GSCDs) from which the ground state constants may be obtained. Our GSCDs cover a considerable range of quantum numbers and the GSCDs from the two bands complement each other. From the a-type component of the n12 band, GSCDs with rather high J and K c are available, but without much information on asymmetry splitting because of the weakness of the asymmetry split lines. However, from the c-type band n18, we have many assignments for the lower K c quantum numbers, over a large J-range which supply important data on the asymmetry splitting. We have therefore performed a simultaneous GSCD analysis on the n12 and n18 bands using a total number of 990 GSCDs formed from mainly unblended lines selected from these bands. The differences reach 65 in J, 64 in K c , and 43 in K a . The model used for our ground state fit was the full asymmetric rotor Hamilton of Watson in A reduced form using IIIr representation (6). The fit was weighted according to the number of transitions sharing a common upper level. For such a group of transitions, each GSCD which might be formed was included and given a weight equal to the reciprocal of the number of GSCDs in the group. The final results are summa-
rized in Table 3, and the extracted ground state rotational constants are summarized in Table 4. The present theoretical equilibrium structure values are given in column 2 for comparison. All the experimental rotation constants and three of the quartic centrifugal distortion constants could be determined with significance. The standard deviation of fit was 0.00077 cm21. Eighteen of the GSCDs have residuals above 0.00180 cm21, and no residual has a numerical value exceeding 0.00220 cm21. The nondiagonal constants d J and d K remained indeterminate. Table 3 also includes the upper state spectroscopic constants for the n12 and n18 bands. These constants have been obtained from separate analyses of the n12 and n18 upper state energies. These energies were derived from the observed wavenumbers by addition of the proper ground state energies as calculated from the constants in Table 3, column 1. For both upper state analyses, the A-reduced Watson Hamiltonian in IIIr-representation was used. As for the ground state, we were not able to determine dJ and dK. Due to the limited number of asymmetry split transitions observed in the n12 band, the parameter ( A 2 B)/2 is considerably less
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HIGH-RESOLUTION IR SPECTROSCOPY OF 1,2,4-TRIAZINE
precisely determined for the n12 level when compared with the n18 level. For C (and DK) the situation is the opposite, because assignments for high Kc are available for n12. In the process of assignment, we observed no crossings due to other vibrational levels, and in the upper state fits, we have left out only observed lines which clearly were of poor quality or obviously overlapped. Since the upper state rotational and centrifugal distortion constants obtained are close to their ground state values, we may certainly conclude that the n12 and n18 bands of 1,2,4-triazine are free from strong perturbations. In the upper part of Figs. 4 – 6, simulations of part of the spectra are shown as stick diagrams. They show the details of cluster structures and bandhead formations. These simulations have been obtained from the program system developed by Pickett (7) using the constants in Table 3. A complete list of all the observations used in the upper state fits, together with the residuals, is given as supplementary data for this article. 6. DISCUSSION
The deviations of the experimental wavenumbers from the simulated data are very small, with a standard deviation of fit well below 1023 cm21; this indicates the high quality of the present moments of inertia data. In a subsequent paper, we hope to extend the measurements to other vibration frequencies. The ground state rotational constants obtained from the present analysis are summarized in Table 4, together with those predicted from the ab initio study of the equilibrium structure. The agreement between the theoretical and experimental constants is very acceptable, bearing in mind that the latter is an
TABLE 3 Spectroscopic Constants Obtained from Analysis of the 1043- and 769-cm21 Bands of 1,2,4-Triazine
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TABLE 4 Ground State Rotational Constants and Inertial Defects for 1,2,4-Triazine
a
Uncertainties quoted are one standard error. From the present equilibrium structure. c For the conversion factor \2/2 the value 16.857630 uÅ2 cm21 has been used. d Inertia defect D 5 I c 2 I a 2 I b . b
equilibrium structure. This strongly suggests that the similarity in several of the ring bond lengths is indeed real. The observed inertial defect (Table 4) is a variable not directly determined from the equilibrium structure. The vibration frequencies determined in the harmonic approximation are very dependent upon the size of basis set and the methodology (self-consistent field uncorrelated, or Moller–Plesset second-order correlation). The large triple-zeta valence 1 polarization with MP2 reduces the differences from experiment to about 50 cm21 or less, with a near linear correlation; although this correlation is dependent upon the symmetry order of frequencies being identical, most of the bands are reasonably well separated. ACKNOWLEDGMENTS We thank the Space Sciences Department at the Rutherford Appleton Laboratory and EPSRC for the generous provision of instrument and staff time, the SERC (now EPSRC) for purchase of a DEC AXP Alpha 3600 workstation, and Edinburgh University for the generous provision of time on the Cray T3D computer. We also thank Dr. M. McPhail (University of Strathclyde, Glasgow) for a copy of the Pickett program and data files, and Bruker UK for the loan of additional frequency range equipment.
REFERENCES
Simultaneous ground state analysis of the 1043 and 769 cm21 bands. b Upper state analysis of the 1043 cm21 band. c Upper state analysis of the 769 cm21 band. d All constants are given in cm21. Uncertainties quoted are one standard error. e Number of observations. f Standard deviation of fit, (cm21). a
1. W. W. Paudler and J. M. Barton, J. Org. Chem. 31, 1720 –1722 (1966). 2. W. W. Paudler and T-K. Chen, J. Heterocyc. Chem. 7, 767–771 (1970). 3. M. H. Palmer, I. C. Walker, M. F. Guest, and M. R. F. Siggel, Chem. Phys. 201, 381–391 (1990). 4. H. Neunhoeffer and H. Hennig, Chemische Berichte 101, 3952–3956 (1968). 5. T. Nakagawa and J. Overend, J. Mol. Spectrosc. 50, 333–348 (1974). 6. J. K. G. Watson, “Vibrational Spectra and Structure” (J. Durig, Ed.), p. 1– 89, Elsevier, Amsterdam, 1977. 7. H. M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991).
Copyright © 1998 by Academic Press
192, 331–337 (1998)
MS987707
Structure and High-Resolution IR Spectroscopy of 1,2,4-Triazine Vapor Michael H. Palmer,*,1 Robert R. J. Maier,* Flemming Hegelund,† and David A. Newnham‡ *Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland, UK; †Department of Chemistry, Aarhus University, DK-8000, Aarhus-C, Denmark; and ‡Space Science Department, Rutherford Appleton Laboratory, Chilton, Oxford OX11 0QX, England, UK Received May 14, 1998; in revised form June 29, 1998
The high-resolution infrared bands centered at 769 and 1043 cm21 for 1,2,4-triazine C3H3N3 have been analyzed, using the full asymmetric rotor Hamilton of Watson in A-reduced form using IIIr -representation. All the ground state rotation constants and three of the quartic centrifugal distortion constants could be determined with high precision from a simultaneous analysis of the two bands. The standard deviation of fit was 0.00077 cm21. The upper state constants for the two bands have been determined with similar precision. The ground state rotational constants obtained from the present analysis are very similar to those predicted from the ab initio study of the equilibrium structure. This strongly suggests that the similarity in several of the ring bond lengths, predicted in the latter, is indeed real. © 1998 Academic Press 1. INTRODUCTION
1,2,4-Triazine(I) is one of the least studied of the heteroaromatic (azine) series C6 –nH6 –nNn, where the known compounds have n 5 0–4. This is because the ring system is readily hydrolyzed, particularly in the presence of moisture and acids (1, 2). Recently, we reported a wide-ranging study of the excited states of I, by combined theoretical and experimental (UV 1 VUV and electron impact spectroscopy) methods (3). The equilibrium structure formed a basis for the theoretical calculations. There is no structural information available for this molecule. We have been unable to obtain a single crystal x-ray diffraction structure, owing to crystal twinning. The three 14N centers and asymmetric rotor structure make microwave spectral determination complex. The theoretical structures obtained by SCF or MP2 methods with a variety of basis sets consistently obtained all ring bonds except the C5C6 bond near 1.33 Å. This makes an electron diffraction study somewhat problematic. The purpose of the present investigation, is to present an analysis of two bands in the high-resolution infrared spectrum, one centered at 1043 cm21 and one at 769 cm21, and to compare the ground state rotational constants with theoretical data. 2. THEORETICAL STRUCTURE AND FORCE FIELD
1,2,4-Triazine, C3H3N3 is thought to be planar, as are the other known azines. A series of basis sets were used in the previous investigation (3), to determine the theoretical equilibrium structure, assuming a planar molecule with C s symmetry. The largest ab initio basis set was triple-zeta in the valence 1
To whom correspondence should be addressed (E-mail: m.h.palmer@ ed.ac.uk).
shell with polarization functions and included Moller–Plesset correlation to second order (TZVP 1 MP2). The final structure, used here, shows 4 ring bonds in the range 1.330 –1.350 Å, with only the C5–C6 bond differing at 1.394 Å (see Fig. 1). The dipole moment lies very close to the N2–C5H axis, which in turn lies at 44.5° ( um a ) to the a axis; thus m a and m b are nearly equal (1.957 and 1.932 D, respectively). The equilibrium structure of the molecule is thus a moderate oblate asymmetric rotor, with an asymmetry parameter k 5 0.72. The harmonic force field predictions of vibration frequencies, expressed in wavenumbers, and integrated absorption intensities are given in Table 1; these are determined by analytic derivatives at the equilibrium geometry of the TZVP 1 MP2 determined structure. In the present work, we have tried to assign the fundamentals of 1,2,4-triazine from the infrared and Raman survey spectra, on the basis of the predictions in Table 1. However, to date, we have not been able to obtain all of the necessary IR data at low frequencies; thus we have not been able to establish an unambiguous assignment, although alignment of the two lists of experimental and theoretical data is one possibility. Overall, we have therefore decided not to include a comparison of the theoretical and experimental data in the Table. In Table 1, the molecule lies in the center-of-charge system (x, y plane), where the y axis is effectively identical to the C3PC6 direction. Fifteen of the normal vibrations, n1–n15, are in-plane, being totally symmetric ( A9). The remaining six, n16–n21, are nonplanar corresponding to A0 symmetry. The totally symmetric vibrations give rise to a,b-hybrid bands, for which the strongest transitions are governed by the symmetric top rotational selection rule DKc 5 61. The vibrations of A0 symmetry yield pure c-type bands whose strongest transitions are characterized by the selection rule DKc 5 0.
331 0022-2852/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
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TABLE 1 Calculated Harmonic Frequencies and Integrated Absorption Intensities
FIG. 1. Equilibrium structure for 1,2,4-triazine showing bond lengths and angles in the inertial axis frame.
3. EXPERIMENTAL SECTION
The compound, prepared by previously reported procedures (1, 4), had spectra very similar in appearance to those of previous studies (3), and in particular, the infrared spectrum in the gas phase shows bands which are close to those reported for the liquid film (1, 4). Laboratory measurements of the midinfrared absorption spectra of 1,2,4-triazine vapor were made using a Bruker IFS 120HR high-resolution Fourier transform spectrometer (FTS), in the Molecular Spectroscopy Facility at the Rutherford Appleton Laboratory. It used a globar broadband IR source, KBr/Ge beam splitter, and liquid nitrogen– cooled mercury cadmium telluride (MCT) detectors. The FTS instrument resolution (R) is defined as R 5 0.9/(MOPD), where MOPD is the maximum optical path difference, which corresponds to the full width at half-maximum (FWHM) with the use of triangular apodization. Sampling fold points and optical and electronic filters were selected to eliminate aliasing and reduce detector nonlinearity effects which would otherwise lead to errors in the recorded spectra. The band-pass optical filters were mounted close to a focus point in the optical beam between the interferometer and gas cell and tilted to reduce optical “channeling” interference effects in the recorded spectra. Throughout the measurements all optical paths inside the FTS were evacuated by a turbomolecular vacuum pump to pressures lower than 1024 hPa, reducing absorption by atmospheric gases in the optical path to negligible levels.
The synthesized sample of 1,2,4-triazine was purified by vacuum distillation and the vapor transferred to a 26.1-cm optical path length stainless steel gas cell via clean Pyrex glass/PTFE vacuum lines. Sample pressures were measured using a calibrated 10-Torr Baratron capacitance gauge (MKS Type 390). The gas cell was fitted with KBr optical windows and thermistor temperature sensors which were attached in thermal contact with the exterior of the cell. Spectra were recorded under the measurement conditions listed in Table 2. Recorded interferograms were transformed using Mertz phase correction and weak Norton–Beer apodization. Under the measurement conditions of each sample measurement, background spectra were recorded at lower spectral resolution with the gas cell evacuated, interpolated onto a wavenumber grid matching that of the sample spectra, and ratioed with sample spectra to give transmittance spectra. The absolute wavenumber scale of the spectra was that determined from a calibration of the internal FTS HeNe laser. The accuracy of this calibration was checked by comparing
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TABLE 2 Instrument Settings
HIGH-RESOLUTION IR SPECTROSCOPY OF 1,2,4-TRIAZINE
333
FIG. 2. A survey spectrum of the n12 (or n13) band of 1,2,4-triazine around 1043 cm21. The central Q branch of the strong a-type band component of n12 is observed at 1043.5 cm21.
line-position measurements of standard calibration gases with literature values. The center positions of the absorption lines were obtained from the raw data, by use of the ORIGIN software. Raw spectra were smoothed three to five times in succession using a Savitzky–Golay five point smoothing routine before applying the peak–find function of ORIGIN. The FWHM and peak position of several peaks were monitored after each smoothing operation, and smoothing was applied just until peak broadening started for prominent peaks. For minor peaks a small amount of peak broadening of up to 0.0065 cm21 was permitted, to facilitate a more accurate and efficient run of the peak–find routine. The effect of the smoothing routine on the peak position was a fraction of the FWHM and therefore negligible. The peak–finding was gradually refined to include increasingly minor features until typically 80 –100 peak positions per wavenumber around the 769- and 1043-cm21 bands were identified. Features located on the shoulders of prominent peaks were not recognized automatically and have been par-
tially excluded, as they might lead to a potentially strong shift in peak position. The survey scans of the two bands studied in the present work are shown in Figs. 2 and 3. 4. ASSIGNMENT OF SPECTRA
The harmonic force field predictions suggest that the 1043cm21 band may be assigned either as n 12 ( A9) or n 13 ( A9). For both fundamental bands, the a-type component is predicted to be ca. three times stronger than the b-type component. Since no strong central Q branch is expected for a b-type component, this prediction is in good accordance with our observation of such a Q branch in the band. However, at present we are not able to choose between the two possible assignments above for the 1043-cm21 band, and we arbitrarily assign it as n 12 ( A9). The n12 band appears as a typical a-type band of a near oblate planar asymmetric top molecule. Intense P P- and R Rbranch clusters spaced approximately ( A 1 B)/ 2 ; 0.21 cm21, dominate the spectrum. Each cluster consists of a series
FIG. 3. A survey spectrum of the n18 (or n19) band of 1,2,4-triaziine around 769 cm21. The Q branch of the strong c-type band n18 is observed at 768.75 cm21.
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FIG. 4. Part of the P P branch of the n12 (or n13) band of 1,2,4-triazine at 1043 cm21. The assignment of P P-cluster structure for 2J0 2 K 0c 5 21 2 24 is shown. Unresolved R Q branches with K 0c 5 20 2 23 are also indicated. The upper part of the figure shows a stick simulation of the region using the constants from Table 3.
of lines for which 2J0 2 K 0c ) is a constant value. Each individual line in a cluster may be characterized by ( J0, K 0c ) 5 ( J 2 t, J 2 2t), where t 5 0, 1, 2, 3, . . . , J/ 2, or ( J 2 1)/ 2 for J even or odd, respectively. The strongest line within each cluster has t 5 0; i.e., it is R R J ( J) or P P J ( J), and the intensity decreases with increasing t. For high t, lines are split due to the molecular asymmetry, and they become difficult to observe in the spectrum due to their low intensity. The assignment was initiated using the method of ground state combination differences (GSCDs) based on the rotational constants calculated from the present prediction of the equilibrium structure. The rotational ground state constants were iteratively improved during the assignment process. The assignment was facilitated by making use of computer assisted Loomis–Wood plot type diagrams, where ordering of the transitions according to upper state energies, to take into account GSCDs, was originally proposed by Nakagawa and Overend (5). In Figs. 4 and 5, the assignments of parts of the P P and R R branches are shown. In the P P branch, a series of sharp unresolved R Q branches appear. The assignment to K c of some of them is also indicated in Fig. 4. Correspondingly, P Q branches are observed in the R R-branch region. However, in this case the
Q branches are broader, and they are not so obvious in the spectrum, as may be seen from Fig. 5. In the band center region near 1043.5 cm21, the molecular asymmetry gives rise to an intense Q-branch structure consisting of P Q and R Q branches for low K c . This is a characteristic feature of a-type bands of near oblate asymmetric tops. However, the lines in this region are so congested, that no assignments of individual transitions are possible. Our final assignment of the n 12 ( A9) band comprises P P- and R R-branch transitions up to J 5 70 and K c 5 66. Asymmetry splitting is observed for K c , 8. The asymmetry split lines are weak, and only short series of such lines are observed. The split lines follow a-type band selection rules; i.e., DK a is even. We also searched for asymmetry split lines in the b component of n12, i.e., lines with DK a odd, but they were too weak to be identified with certainty. The survey spectrum of the 769-cm21 band is shown in Fig. 3. Two Q branches appear near the band center; a strong Q branch at 768.75 cm21, and a weaker one at 770.05 cm21. From our harmonic force field prediction, three fundamental bands may appear in this region: n 14 ( A9), n 18 ( A0), and n 19 ( A0). The P and R branches belonging to the strong Q branch show cluster structure with groups of lines spaced ca.
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FIG. 5. Part of the R R branch of the n12 (or n13) band of 1,2,4-triazine at 1043 cm21. The assignment of R R-cluster structure for 2J0 2 K 0c 5 16 2 20 is shown. The upper part of the figure shows a stick simulation of the region using the constants from Table 3. The P Q branches which are widespread are marked on the simulation. They are only weakly discernable in the observed spectrum.
( A 1 B) ; 0.42 cm21. This indicates that the band is of c type; the observed cluster structure arises from more or less resolved K c structure of individual Q P- and Q R-branch lines, each characterized by J0. At present we are not able to decide whether the 769-cm21 c-type band is n18 or n19, and we arbitrarily assign it as n 18 ( A0). No rotational P- and R-branch structure belonging to the weaker Q branch at 770.05 cm21 has been observed. We are therefore not able to assign this Q branch to a fundamental vibration with certainty. However, our force field predictions suggest that it might be due to n 14 ( A9). A cluster in the n18 band consists of the individual K c components of the Q P- or Q R-branch lines for which J0 is a constant value. The strongest lines within each cluster have K 0c 5 0, and the intensity diminishes slowly with K 0c increasing. This is quite different from the situation for the a-type band clusters discussed above, where the strongest line was for K 0c 5 J0. Furthermore, for low K c there is systematic overlap of lines because energy levels J, K a , K c and J, K a , K c 1 1 with K a 5 J 2 K c become systematically degenerate for high J and low K c . This means, e.g., that the line series Q R K ( J) and Q R K11 ( J) with K a 5 J 2 K overlap (K 5 K c ). Furthermore, the lines in each cluster converge to a bandhead for high K c .
This bandhead formation is particularly prominent in the Q Rbranch region. For the Q P branch, the bandheads are clearly observable too, although they are somewhat broader. In Fig. 6, a small part of the assignment of the Q R branch of n18 is shown. In the central Q Q branch no assignments are possible because of the high degree of blending of lines. Our final assignment of the n18 band includes Q P- and Q R-branch lines up to J 5 70 and K c 5 33. Asymmetry splitting is observed for K c , 13. In contrast to the a-type band n12, the c-type band n18 shows long series of asymmetry split lines of moderate intensity. 5. GROUND AND UPPER STATE SPECTROSCOPIC CONSTANTS
To our knowledge, no previous microwave or high-resolution infrared study has been published on the ground state of 1,2,4-triazine. Precise ground state rotational and centrifugal distortion constants of this molecule have therefore not been determined before by spectroscopic methods. Our assignments of the rotational structure of the n12 and n18 bands of 1,2,4-triazine yield a number of ground state combi-
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FIG. 6. Part of the Q R branch of the c-type band n18 (or n19) of 1,2,4-triazine around 1043 cm21. The assignments to K c of the fine structure of the Q R(29) and part of the Q R(28) branches are shown. The value of the K a quantum number is given as a parity. For plus parity, K a 5 J 2 K c ; for negative parity, K a 5 J 2 K c 1 1. For low K c , the components are pairwise coincident. See text. For Q R(28), splitting is observed for the pair K c 5 5 2 , 4 1 . For K c . 12, lines are not resolved, and a stick diagram of Q R(29), using the constants of Table 3, shows a prediction of K c splitting and bandhead formation.
nation differences (GSCDs) from which the ground state constants may be obtained. Our GSCDs cover a considerable range of quantum numbers and the GSCDs from the two bands complement each other. From the a-type component of the n12 band, GSCDs with rather high J and K c are available, but without much information on asymmetry splitting because of the weakness of the asymmetry split lines. However, from the c-type band n18, we have many assignments for the lower K c quantum numbers, over a large J-range which supply important data on the asymmetry splitting. We have therefore performed a simultaneous GSCD analysis on the n12 and n18 bands using a total number of 990 GSCDs formed from mainly unblended lines selected from these bands. The differences reach 65 in J, 64 in K c , and 43 in K a . The model used for our ground state fit was the full asymmetric rotor Hamilton of Watson in A reduced form using IIIr representation (6). The fit was weighted according to the number of transitions sharing a common upper level. For such a group of transitions, each GSCD which might be formed was included and given a weight equal to the reciprocal of the number of GSCDs in the group. The final results are summa-
rized in Table 3, and the extracted ground state rotational constants are summarized in Table 4. The present theoretical equilibrium structure values are given in column 2 for comparison. All the experimental rotation constants and three of the quartic centrifugal distortion constants could be determined with significance. The standard deviation of fit was 0.00077 cm21. Eighteen of the GSCDs have residuals above 0.00180 cm21, and no residual has a numerical value exceeding 0.00220 cm21. The nondiagonal constants d J and d K remained indeterminate. Table 3 also includes the upper state spectroscopic constants for the n12 and n18 bands. These constants have been obtained from separate analyses of the n12 and n18 upper state energies. These energies were derived from the observed wavenumbers by addition of the proper ground state energies as calculated from the constants in Table 3, column 1. For both upper state analyses, the A-reduced Watson Hamiltonian in IIIr-representation was used. As for the ground state, we were not able to determine dJ and dK. Due to the limited number of asymmetry split transitions observed in the n12 band, the parameter ( A 2 B)/2 is considerably less
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HIGH-RESOLUTION IR SPECTROSCOPY OF 1,2,4-TRIAZINE
precisely determined for the n12 level when compared with the n18 level. For C (and DK) the situation is the opposite, because assignments for high Kc are available for n12. In the process of assignment, we observed no crossings due to other vibrational levels, and in the upper state fits, we have left out only observed lines which clearly were of poor quality or obviously overlapped. Since the upper state rotational and centrifugal distortion constants obtained are close to their ground state values, we may certainly conclude that the n12 and n18 bands of 1,2,4-triazine are free from strong perturbations. In the upper part of Figs. 4 – 6, simulations of part of the spectra are shown as stick diagrams. They show the details of cluster structures and bandhead formations. These simulations have been obtained from the program system developed by Pickett (7) using the constants in Table 3. A complete list of all the observations used in the upper state fits, together with the residuals, is given as supplementary data for this article. 6. DISCUSSION
The deviations of the experimental wavenumbers from the simulated data are very small, with a standard deviation of fit well below 1023 cm21; this indicates the high quality of the present moments of inertia data. In a subsequent paper, we hope to extend the measurements to other vibration frequencies. The ground state rotational constants obtained from the present analysis are summarized in Table 4, together with those predicted from the ab initio study of the equilibrium structure. The agreement between the theoretical and experimental constants is very acceptable, bearing in mind that the latter is an
TABLE 3 Spectroscopic Constants Obtained from Analysis of the 1043- and 769-cm21 Bands of 1,2,4-Triazine
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TABLE 4 Ground State Rotational Constants and Inertial Defects for 1,2,4-Triazine
a
Uncertainties quoted are one standard error. From the present equilibrium structure. c For the conversion factor \2/2 the value 16.857630 uÅ2 cm21 has been used. d Inertia defect D 5 I c 2 I a 2 I b . b
equilibrium structure. This strongly suggests that the similarity in several of the ring bond lengths is indeed real. The observed inertial defect (Table 4) is a variable not directly determined from the equilibrium structure. The vibration frequencies determined in the harmonic approximation are very dependent upon the size of basis set and the methodology (self-consistent field uncorrelated, or Moller–Plesset second-order correlation). The large triple-zeta valence 1 polarization with MP2 reduces the differences from experiment to about 50 cm21 or less, with a near linear correlation; although this correlation is dependent upon the symmetry order of frequencies being identical, most of the bands are reasonably well separated. ACKNOWLEDGMENTS We thank the Space Sciences Department at the Rutherford Appleton Laboratory and EPSRC for the generous provision of instrument and staff time, the SERC (now EPSRC) for purchase of a DEC AXP Alpha 3600 workstation, and Edinburgh University for the generous provision of time on the Cray T3D computer. We also thank Dr. M. McPhail (University of Strathclyde, Glasgow) for a copy of the Pickett program and data files, and Bruker UK for the loan of additional frequency range equipment.
REFERENCES
Simultaneous ground state analysis of the 1043 and 769 cm21 bands. b Upper state analysis of the 1043 cm21 band. c Upper state analysis of the 769 cm21 band. d All constants are given in cm21. Uncertainties quoted are one standard error. e Number of observations. f Standard deviation of fit, (cm21). a
1. W. W. Paudler and J. M. Barton, J. Org. Chem. 31, 1720 –1722 (1966). 2. W. W. Paudler and T-K. Chen, J. Heterocyc. Chem. 7, 767–771 (1970). 3. M. H. Palmer, I. C. Walker, M. F. Guest, and M. R. F. Siggel, Chem. Phys. 201, 381–391 (1990). 4. H. Neunhoeffer and H. Hennig, Chemische Berichte 101, 3952–3956 (1968). 5. T. Nakagawa and J. Overend, J. Mol. Spectrosc. 50, 333–348 (1974). 6. J. K. G. Watson, “Vibrational Spectra and Structure” (J. Durig, Ed.), p. 1– 89, Elsevier, Amsterdam, 1977. 7. H. M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991).
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