J. CHEM. SOC. FARADAY TRANS., 1993, 89(1), 43-48 43

Vibrational Spectrum of Pyrazinecarbaldehyde Vicente Botella,* Alfonso Hernandez-Laguna and Yves G. Smeyers lnstituto de Estructura de la Materia (C.S.I.C), CISerrano 119,28006 Madrid, Spain Maria Jose Martin-Delgado, Maria Josefa Macedo and Maria Isabel Suero Departamento de Fisl'ca , Universidad de Extremadura , E-06071 Badajoz, Spain

~~ ~

Pyrazinecarbaldehyde has been synthesized and its infrared spectrum recorded. This spectrum has been assigned on the basis of C, symmetry. A6 initio calculations in 6-31G split-valence basis set have been per- formed and the infrared spectrum has been calculated at the theoretical equilibrium geometry of the trans conformer. A scaling quantum-mechanical procedure has been done to ascertain the experimental assignments. As a result, scaled-wave numbers and assignments agree fairly well with the experimental ones. Scaling factors are shown for use in other pyrazine derivatives.

Spectroscopical studies of pyrazine derivatives, such as pyrazine-2-carboxylic acid,' pyra~ine-2-carbonitriIe,~ pyrazine-2-~arboxamide,~*~ and pyrazine-2,3-dicarb~xamide~ have been reported. Vibrational analysis of the infrared and Raman spectra of these derivatives was performed by using "N and D isotopic substitutions.

Some of the compounds studied have pharmacological and industrial applications : pyrazine-2-carboxamide acts as an anti tuberculos tatic agen t,6-9 and pyrazinecarbonitrile as a fungicide agent. "9 ' '

With the exception of pyrazinecarboxamide, experimental and theoretical structural studies of these compounds have not been performed. It is, therefore, of interest to analyse the vibrational spectrum of pyrazinecarbaldehyde, which is the simplest framework of these types of molecule. Pyra- zinecarboxaldehyde was first described as a transient chemi- cal product arising from the in situ preparations of 2,4- dinitrophenylhydrazine and thiosemicarbazone by means of the MacFyden-Stevens reaction.6 Pyrazinealdehyde is a very unstable compound and decomposes in ca. half an hour, so that it has to be stored in solution. Even though this mol- ecule was described in the early 1950s, we have not found any reference to its molecuiar structure in the literature.

In this paper, the synthesis, infrared spectrum and a theo- retical analysis of pyrazinecarbaldehyde are reported. Its sta- bility features make the isotopic substitution a very difficult task, and the infrared spectrum has been analysed via ab initio computational methods. Theoretical and experimental research concerning the large-amplitude motions (far-infrared region), is currently in progress.

Experimental Pyrazinecarbaldehyde is sensitive to light, it has a low melting point (31-33"C), and its boiling point is 57-58°C at a pressure of 6 mm Hg.? This product is easily transformed in half an hour in other pyrazine derivatives and it undergoes the Cannizaro reaction, giving pyrazinecarboxylic acid and pyrazinemethanol as products.

The Rutner and Spoerri synthesis procedure'2 has been employed, starting from the pyrazinecarboxylic acid to obtain the crystallized methylic ester. This compound is reduced with AlLiH, dissolved in tetrahydrofuran, and finally acidification with acetic acid gives pyra- zinecarboxaldehyde. The compound was isolated by pre- parative thin-layer chromatography. The quantity obtained by this separation is rather small, but sufficient for recording an infrared spectrum.

The infrared spectrum of the liquid sample was recorded with a dispersive Perkin-Elmer spectrophotometer model 883 in the range 4000-200 cm-', and with a Fourier-transform Nicolet model 20DX in the range 4000-400 cm-'. In this work we present only the range of frequencies common to both spectrophotometers ; the frequencies around 400 cm- ' are far from the lowest limit of the dispersive instrument, so that they are reliable. To calibrate the Perkin-Elmer we have used standard^.'^ The resolving power of both spectropho- tometers is 0.5 cm-'.

The experimental infrared wavenumbers, intensities and proposed assignments are shown in Tables 1 and 2.

Theoretical Met Rods It is well known that RHF ab initio calculations with split- valence basis sets and larger, usually lead to harmonic force fields and fundamental vibrational transitions that are quali- tatively well behaved, but far from the experimental result^,'^.'^ owing to the lack of electronic correlation in the wavefunction and of a consideration of anharmonicity.

We have used the Monstergauss p r ~ g r a m t ' ~ ? ' ~ and Gaussian 8618 to carry out the calculations. The theoretical infrared spectrum was obtained by using Gaussian 86 which has a procedure for obtaining analytical second derivatives of the electronic energy, and permits the direct acquisition of the harmonic force field and fundamental wavenumbers. Mon- stergauss was used to determine the conformations of minimum energy. Davidon's optimally conditioned method was employed to locate the rninima.lg

Even if we use 'state of the art' theoretical calculations, the obtained spectrum cannot compete against the experimental one; the wavenumbers calculated are not good enough for a direct assignment, but the deviations from experiment are systematic rather than random.' Therefore, some procedure to correct the frequencies is needed.

As the theoretical wavenumbers are higher than the experi- mental ones, several methods have been proposed to scale down the harmonic force field in order to reduce the discrep- ancy between theoretical and experimental wavenumbers. First, a scaling only in the diagonal force constants was tried,20 and next, Pulay et aL2' proposed a better method based on the equation:

t In addition to the Gaussian 70 integral and SCF subroutines, the program ,incorporates analytical gradients and automatic opti- mizations with and without constraints. t 1 mmHg z 133.322 Pa.

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44 3. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

Table 1 In-plane experimental bands (cm-') and assignments, 6-31G and scaled wavenumbers, and M matrix coefficients" (20.10) of the internal coordinates (see Table 4)

V intensity experimental assignment 6-3 1G scaled coefficient of M

3074 3060 2980 2840 1722 1576 1523 1438 1 404 1366 1309 1238 1166 1136 1048 1018 839 688 65 5 460

W

W W

W

vs W

vw m m W vs W m

m

m

S

S

W W

vvw

3440 3433 3413 3276 1906 1776 1742 1644 1567 1516 1438 1389 1292 1225 1165 1128 934 757 68 1 516 237

3048 3041 3024 2840 1722 1593 1559 1465 1397 1367 1285 1234 1158 1093 1042 1033 84 1 696 636 460 20 1

0.841 , , 0.431, + 0.401,, + 0.161,,

0.501, + 0.491,, - 0.991 , , 0.901,

0.191, + a.iiz,fz, + 0.111,~ - 0.261, + 0.161, + 0.231, + 0.121,

0.151, + 0.361,6 + O.JZ,, 0.101, + 0.151, + 0.401,, + 0.11Z2,,

0.241, + 0.26Z1, + a.ioz,, + o.ioz,, 0.121, + 0.141, + 0.211, + O.l2Z,, + 0.281,,

0.421, + 0.211, + 0.131, + 0.11Z16 0.151, + 0.151, + 0.491,,

0.221,, + 0.161,, + 0.151,, + 0.201,,

- 0.361, + 0.241,, + 0.121,,

0.451,, + 0.371,, 0,1312 + 0.1116 + 0.33116 + 0.35118

0.201, + 0.241, + 0 x 1 , + 0.211,

0.211, + 0.181, + 0.251,,

- 0.131,, + 0.691,,

- 0.711,, + 0.141,,

" Coefficients of the significant group of internal coordinates are underlined.

where Ftheor and Fscal are the theoretical and scaled force con- stant matrices, respectively, and S , is a diagonal matrix with the scaling factors. As seen, the diagonal force constants Fii scale with the corresponding scaling factor, and the coupling force constants F , scale with the square root of S i and S j .

In order to determine a more accurate harmonic force field it is necessary to refine the values of the force constants. However, only one isotopic derivative has been recorded and few experimental frequencies are available to undertake this task.

Results and Discussion Analysis and Assignments of the Infrared Spectrum

Since the pyrazinecarboxaldehyde molecular structure is unknown, some considerations about its expected symmetry can be helpful. As this symmetry should be strongly related to the aldehyde conformation, pyrazinecarbaldehyde may belong to the C , or the C , point groups. In order to single out the pertinent point group, comparisons with the a priori similar molecular structure of p~ridine-2-carbaldehyde*~-~~ can be made and ab initio calculations performed. Two planar (cis and trans) conformers have been found for the latter molecule and two similar conformers could be foreseen for the pyrazinecarbaldehyde.

The molecular structure of pyrazinecarbaldehyde has been calculated by a full-geometry optimization at the 6-3 1G level (Fig. 1) and a trans conformer has been found (more details will be shown later on). The trans conformer has the oxygen of the aldehyde group in a trans position with respect to the closest nitrogen of the pyrazine ring. Therefore, the C , point group is more appropriate for the pyrazinecarbaldehyde mol- ecule. Adopting this point group, the 30 normal vibrations are distributed as follows: 21A' + 9A".

To assign the vibrational spectrum, the normal modes have been classified as in-plane and out-of-plane. Within each class there are three groups: (a) vibrations of the pyrazine ring; (b) vibrations of the aldehyde group and ( c ) vibrations of the ring-substituent. In Tables 1 and 2 the wavenumbers for the fundamental transitions are collected and the assignment notation follows that used in the spectrum of pyrazine.26

In-plane Vibrations Vibrations of the Pyrazine Ring. The C-H stretchings are

usually the highest fundamental wavenumber vibrations and generally appear at ca. 3000 cm-'. We have assigned the 3074, 3060 and 2980 cm- ' bands as C-H stretchings. All of them have weak intensities. These assignments agree fairly well with those done in similar compounds [ref. (1-4), and references t h e r e i ~ ~ ] . ~ ~ - ~ '

Among the six stretching ring modes, the 8a, b and 19a, b pairs are vibrations with a wavenumber very similar in all the pyrazine derivatives. The 8a and 8b have been assigned to the 1576 and 1523 cm-' bands, respectively. The vibration 8a has a higher wavenumber than 8b as is usual in heterocycles with nitrogen,29 their difference (53 cm-') being close to that of the monosubstituted pyrazine derivative^.^' The pair of bands 19a, b has been assigned to 1438 and 1404 cm-',

Fig. 1 the trans conformation

Redundant internal coordinates of pyrazinecarbaldehyde in

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respectively. Both of them are very regular bands in the series of pyrazine derivatives. The strong band at 11 36 cm- ' corre- sponds to the mode 14; vring by correlation with pyrazine3' and pyrazinecarbonitrile.' The mode 1 could be assigned to 839 cm- ' band, a value below the wavenumbers of pyrazine (1016 cm-') and 2,5-dimethylpyrazine (860 cm-')31 and just above the wavenumber of 2-methylpyrazine (830 cm- 1).27 All of these differences can be ascribed to the substituent effect.

The C-H deformation vibrations B(CH), have been assigned by correlation with the 2-methylpyrazineZ7 and cor- respond to 1309,1166 and 1048 cm-'.

As regards (jring modes, the following assignments can be made. 12; Bring is not significantly shifted by the substituent effect, and could be assigned to 1018 cm-'. The 6a, b; Bring pair has been assigned to the bands 460 and 655 cm-', respectively. The first is a very weak band, and it has been assigned by correlation with the spectrum of pyra- zinecarbonitrile.2

Vibrations of the Aldehydic Substituent. The C-H stretch- ing of the aldehyde group usually ranges from 2900 to 2700 cm-' 3' We have assigned this vibration to the 2840 cm-' band, whose wavenumber and intensity agree with those found in pyridine-2-carbaldehyde.

The v(C0) band is well known and corresponds to the highest intensity band of the spectrum at 1722 cm-'.

The B(CH) low-intensity mode usually appears at ca. 1400

the 6(CH) mode appears at 1365 and 1387 cm- ', respectively. We have assigned this mode to the 1366 cm- ' band.

The B(C0) mode was found at 649 cm-' in ben~a ldehyde~~ and at 662, 665 and 665 cm- ' in pyridine-2-, -3- and -4-carbaldehyde3 respectively. In pyra- zinecarbaldehyde this mode has been assigned to the 688 cm- ' band.

Ring-substituent Vibrations. The v(CX) mode usually appears at the highest wavenumber of the ring-substituent vibrations. The CX stretching wavenumbers in the mono- substituted benzene derivatives with a light substituent fall within the 1100-1280 cm-' range.26 We have assigned the 1238 cm-' band to this mode.

The B(CX) mode should appear at under 300 cm-'. This band has not been observed because our spectrophotometer does not reach such low wavenumbers.

.

cm- 1 . 3 4 In pyridine-2-carbaldehyde3 and ben~aldehyde,~

Out-ofplane Vibrations Vibrations of the Pyrazine Ring. The three y(CH) modes

have been assigned to the 915, 870 and 775 cm-' bands by correlation with other pyrazine derivatives studied by some of

The ring torsions have been assigned as follows: mode 4 corresponds to the 755 cm-' band by correlation with

pyrazinecarbonitrile' and methylpyrazine." The 16a, b pair has been assigned to two shoulders located at 437 and 400 cm-', respectively. Both of them are somewhat doubtful because of their weak intensity and closeness to the limit of the recording range.

Vibrations of the Aldehydic Substituent. The out-of-plane y(CH) of the aldehyde substituent was located at 1010 cm- ' in b e n ~ a l d e h y d e ~ ~ and at 1006, 1005 and 1005 cm-' for the pyridine-2-, -3-, and -4-~arbaldehydes,~~ respectively (Zwarich et quote 1007, 1010 and 1005 cm-' for the three isomers of pyridinecarbaldehyde), all of them having very weak intensities. Otherwise, in aromatic aldehydes, Colthup3' predicted that the y(CH) mode should appear in the range 975-825 cm-l. In the present molecule, the y(CH) aldehyde mode could be assigned to the 962 cm-' band with weak intensity. However, it could be hidden within the 1018 cm-' high-intensity band. The former band has been assigned to the y(CH) aldehyde mode.

Ring-substituent Vibrations. The out-of-plane y(CX) mode should appear under 300 cm-'. This band has not been observed because of the spectrophotometer employed. For the same reason, the torsional mode does not appear in our record. However, in benzaldehyde this mode was assigned to the 133 cm-' band36 and those at 130,26 130 and 126 cm-' for pyridine-2-, -3- and -4-carba ldeh~de~~ in the liquid state. Miller et have recorded a torsional mode in the gas phase at 111, 110 and 102 cm-' for pyridine-2-, -3- and -4- carbaldehyde, respectively. Therefore, in pyrazinecarbalde- hyde the torsional mode should be found at ca. 110 cm- ' in gas phase and near 130 cm- in the liquid state.

In Table 3, combinations, overtones and difference bands are also shown with their tentative assignments. Since this is primarily a study of pyrazinecarbaldehyde, and because of its stability, the complex synthesis procedure and the small quantity of sample obtained, it is very difficult to procure another kind of spectrum such as isotopic substitution, Raman, trapping in inert matrix. So, we have used quantum- chemical calculations as a complementary technique to aid in the experimental assignment.

Normal-coordinates Calculations and Theoretical Assignments

To assign the fundamental transitions, theoretical calcu- lations on the pyrazinecarbaldehyde normal modes have been performed at the 6-31G level. The theoretical wavenum- bers are shown in Tables 1 and 2, where a scaling was carried out to compare these values with experiment. This procedure basically follows the scaling quantum-mechanical (SQM) method developed by Pulay et al." By so doing, the ab initio equilibrium geometries are empirically corrected in order to

Table 2 internal coordinates (see Table 4)

Out-of-plane experimental bands (cm- ') and assignments, 6-31G and scaled wavenumbers, and M matrix coefficients" (30.10) of the

~ ~~

V intensity experimental assignment 6-31G scaled coefficient of M

962 915 870 775 755 437 400

~

W

W

vw m m

vvw vvw

1163 1145 1107 977 857 547 492 260 132

960 909 8 79 778 714 468 416 225 117

0.151,, + 0.751,, WI,, + 0.631,, U Z , , + 0.761,, 0.421,, + 0.361,,

0.451F-t WZ,,

0.471,, + ti%,, + 0.211,, 0.131,, + 0.671,, + 0.131,,

- 0.901,,

0.931,,

" Coefficients of the significant group of internal coordinates are underlined.

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46 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

Table 3 Combination, overtone and difference bands for pyra- zinecarbaldeh yde

~

V intensity assignment

295 1 2705 1953 1925 1892

1749 1680 1638 1540 1285 1199

S

W

W

W

vw

S W

vw W m m

obtain geometries closer to the experimental values. With this correction, the force constants decrease, thus approaching the experimental ones. In this paper, we have not performed any empirical geometry corrections, since there are no sets of cor- rection parameters for the 6-31G basis set.

In Fig. 1 all the bond lengths and angles are shown in order to define a set of non-redundant internal coordinates which is defined in Table 4 and allows transferable scaling factors to be obtained for similar molecules.38 It is impossible to obtain directly the analytical gradients and second deriv- atives by using either the redundant set of internal coordi- nates (Ri, cti, see Fig. 1, but also Pulay et aL3'), or the non-redundant set of internal coordinates, see Table 4. There- fore, the ab initio calculations have been performed in stan- dard internal coordinates appropriate to the 2 matrix of the LCAO-MO type programs. This set of coordinates is not symmetrically complete, so that it is not possible to obtain the set of Table 4 in a straightforward manner. We have to transform the force field to Cartesian coordinates :

Table 4 Internal coordinate system :' non-redundant coordinates ~~~~~

internal coordinate description li

1, 2, 4, 5 3, 6 7 8 9-1 1 12 13

14

15 16-18 19 20 21 22

23 24

25 26

30 27-29

Cring-Nring stretching Cring-Cring stretching

C - 0 stretching Cring-H stretching CaId-H stretching ring deformation

Cring-Caldehyde stretching

ring deformation

ring deformation Cring-H rocking Cring-Cald rocking C - 0 deformation C-H aldehyde rocking ring torsion

ring torsion ring torsion

aldehyde internal rotation Cring-Cald

Cring-H WagWg C,,,-H wagging

aSee Fig. 1. b ~ 1 = 6 - 1 - 2 - 3 , ~ , = 1 - 2 - 3 - 4 , ..., 7 i =

(i - 1) - i - (i + 1) - ( i + 2); i = 1-6 cyclic. Xi - C, 7ald = - C , - (all dihedral angles).

B, matrix transforms Cartesian to 2-type internal coordi- nates. Afterwards, a final change of coordinates is necessary:

here B, transforms Cartesian to the symmetrically complete set of internal coordinates, see Table 4.

The calculations were performed in two separate runs cor- responding to in-plane and out-of-plane vibrations. Scaling factors have been collected in separate groups, depending on the nature of the internal coordinate involved, see Fig. 1, and they are shown in Table 5. The scaling factors of some inter- nal coordinates are held fixed because they participate prin- cipally in the two lowest frequencies for which there are no experimental measurements. We performed the out-of-plane calculations by varying all the scaling factors and a null devi- ation was obtained, although the values of the scaling factors were not realistic.

In general, the scaling factors for the in-plane modes present values according to the type of ab initio calculation and basis set. The out-of-plane scaling factors are slightly smaller than the in-plane ones. The root-mean-square devi- ations for the in-plane and out-of-plane wavenumbers are close, 18 and 22 cm-'. According to the range of wavenum- bers and the scaling factors obtained, the reliability of the fitting is similar in both cases.

In order to check the experimental assignments, the M matrix was used to identify the contribution of the internal coordinates to the normal modes2' The matrix elements can be written in the form:

and provide the contribution of the Ith internal coordinate to the ith normal mode. The M matrix plays the same role as the potential-energy distribution in the interpretation of normal modes.

The SQM method works similarly to a refinement process to determine force constants:' but with scaling factors. The iterative calculation finishes when a minimum root-mean- square deviation between experimental and scaled wavenum- bers is reached.

I n-plane Vibrations Vibrations of the Pyrazine Ring. The three theoretical

v(CH) stretching modes are in agreement with the experimen-

Table 5 Scaling factors"

internal coordinates scaling factors

1, 2, 4, 5 3, 6 7 8 9, 10, 1 1 12 13, 14, 15 16, 17, 18 19 20 21 22, 23, 24 25 26 27, 28, 29 30

~

0.794 0.807 0.723 0.834 0.785 0.75 1 0.876 0.798 0.667 0.987 0.712 0.695 0.8b 0.8b 0.627 0.685

~~

a Initial values were established at 0.8. correspond to modes without an experimental partner.

Fixed scaling factors which

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 47

tal wavenumbers, see Table 1. The lowest-frequency mode deviates by 44 cm-', which is twice that of the other two modes. The higher-wavenumber mode is closely represented by the I , , internal coordinate and the other two can be con- sidered mainly as a mixing of I , and I , , .

With respect to the six vring modes, the agreement between the experimental and theoretical wavenumbers is not com- pletely satisfactory. In the 8a, b pair, the deviation of 8a is less than that of 8b. The former corresponds to C-C motions, in which I , and 16 participate, and 8b corresponds to a C-N vibration, which is a mixing of the four CN inter- nal coordinates. The coefficients of the M matrix suggest that the theoretical assignment of the 19a,b couple should corre- spond to C~(CH),~,,~ instead of vring modes. Even though there is minimal participation of the vring internal coordinates, the 19a, b modes cannot be confidently assigned to vring. The pyrazine molecule shows 19a, b vring modes with a high par- ticipation of the C?(CH),~,,, internal coordinate^,^' and the sub- stitution of an H by an aldehyde group may cause a substantial change in the participation of internal coordi- nates. Although the mode 14 is clearly a vring (CN), the dis- crepancy between its computed and experimental value is the highest all over the frequency range explored. Mode 1 is found to be a combination of the 1 6 , I , and I14 internal coordinates, in which the C-Cc,,, and the internal ring angles are involved. Therefore, it is difficult to identify this mode as a pure vring.

One of the three 6(CH) ring modes does not agree with experiment. The mode at 1309 cm-' is a pure 6(CH) and the mode at 1166 cm-' is clearly a 6(CH) mode with an impor- tant mixing of vring (C-C). In the theoretical assignment the third mode at 1048 cm-' is a vring because of the high partici- pation of I , and I , , although there is a small participation of

The three ring-deformation modes (12 and 6a, b couple) agree with the experimentally predicted bands. In the 12 and 6b modes, the theoretical assignment confirms the experimen- tal one. However, the 6a mode at 460 cm-I has a high par- ticipation of v(CX), but it has such a low experimental intensity that the assignment could be considered doubtful.

Vibrat ions of the Aldehyde Substituent. All the theoretical vibrations with the aldehydic substituent fit the experimental wavenumbers very well. The two stretching modes v(CH) and v(C0) have elements with the highest values of the M matrix corresponding to I , , and I , , respectively. 6(CH) at 1366 cm-' is a combination of I,, and I , , internal coordinates, both of them being aldehyde angles. Finally, d(C0) is com- posed of I , , and I , , , and some participation of I , , and I , , internal coordinates.

Ring-substituent Vibrat ions. The theoretical v(CX) band at 1238 cm-' shows an important participation of the Z l 3 inter- nal coordinate and therefore it could be regarded as v(CX) contaminated with 6 ring.

The theoretical mode at 201 cm-' does not have any experimental partner and it is possible to assign it to 6(CX).

z 1 6 .

Out -o fp lane Vibrat ions Vibrat ions ofthe Pyrazine Ring. The three y(CH) modes fit

very well. The mode corresponding to the 870 cm-' band is mainly composed of the internal coordinate I , , , and the other two modes at 915 and 775 cm-' are mainly a com- bination of I , , and I , , .

The three torsional modes are in worse agreement than the y(CH) modes. The deviation from experiment of the 755 cm-' band is large; however, the elements of the M matrix show this band as a pure ring torsional mode. The 437 cm-' band also shows a large deviation and corresponds to a ring torsion with participation of the aldehyde wagging group.

This deviation is probably due to the fixed scaling factor for this last internal coordinate, 1 2 6 . It is impossible to include a relaxation condition in the scaling factor because of the two unknown experimental wavenumbers which participate in the scaling process with null weight. If we let the scaling factor vary, these bands would have a destabilizing effect in the scaling procedure for the fitting. As stated above, we made this scaling factor variable, and obtained a final unrealistic value of 0.2967 and null deviation. The 400 cm-' band fits better than the other two and appears to be a pure ring torsion.

Vibrat ions of the Aldehyde Substituent. The y(CH) of the aldehyde group fits very well. This mode is described by I , , with _minor participation of the 1 2 6 internal coordinate.

Ring-substituent Vibrat ions. There are no experimental counterparts for the two ring-substituent vibrations. The scaled theoretical wavenumbers included in the general SQM procedure present one mode which mainly corresponds to the torsion of the aldehyde group, the other being a com- bination of aldehyde torsion, aldehyde ring and aldehyde y(CX) modes.

Summary and Conclusions The pyrazinecarbaldehyde molecule has been synthesized and its infrared spectrum recorded. The fundamental tran- sitions and some overtone and combinations bands have been assigned and compared with the theoretical spectrum computed at the 6-31G level. As a result, the experimental spectrum is consistent with the C, symmetry point group, and the agreement between experimental and theoretical bands is fairly good. Nevertheless, three modes are doubtful and some others are very mixed as the SQM results suggest. In addi- tion, a set of scaling factors for use in pyrazine derivatives is also reported.

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9

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