Vibrational spectra of N-methylpyrazole: an experimental and theoretical study

Spectrochimica Acta Part A 53 (1997) 1383 – 1398

Vibrational spectra of N-methylpyrazole: an experimental and theoretical study Jose´ M. Orza a,*, Otilia Mo´ b, Manuel Ya´n˜ez b, Jose´ Elguero c b

a Instituto de Estructura de la Materia, CSIC, Serrano 123, E-28006 Madrid, Spain Departamento de Quı´mica, C-9 Uni6ersidad Auto´noma de Madrid, Cantoblanco, E-28049 Madrid, Spain c Instituto de Quı´mica Me´dica, CSIC, Juan de la Cier6a 3, E-28006 Madrid, Spain

Received 10 January 1997; accepted 4 February 1997

Abstract The gas-phase infrared spectrum of N-methylpyrazole was measured in the range 5000 – 500 cm − 1 and with a resolution of 0.5 cm − 1. Its Raman spectrum was obtained in the condensed-phase in the range 3500 – 100 cm − 1 and with a resolution of 2.7 cm − 1. The corresponding depolarization ratios were also measured. Theoretical information on harmonic vibrational frequencies, infrared and Raman intensities and depolarization ratios was obtained by means of ab initio and density functional theory approaches. The former were performed at the MP2/6-31G** level and the latter at the B3LYP/6-31G** level. This information was useful in the assignment of the different fundamentals. In general the agreement theory-experiment was very good, with the only exception of the infrared and Raman intensities of some transitions. © 1997 Elsevier Science B.V. Keywords: N-methylpyrazole; Raman spectrum; Vibrational spectra

1. Introduction Pyrazole and its derivatives have received a great deal of attention. They have a very versatile chemistry and constitute the active moieties of several biochemical systems as well as the ligands of many organometallic compounds [1]. The infrared spectrum of pyrazole itself 1 has been the subject of many different studies, the most complete being those of Tabacik et al. [2] on the IR spectra of pyrazole and seven deuterated derivatives in the gas and in Ar matrices (for other papers on the same group, see [3]), Majoube [4,5]

on higher resolution IR gas spectra of pyrazole and deuterated derivatives (and also as CCl4 solutions and polycrystalline solids) and, more recently, Durig et al. [6] on the Raman spectrum of liquid and polycrystalline pyrazole.

* Corresponding author. Tel.: +34 1 5616800; fax: + 34 1 5855184; e-mail: [email protected] 1386-1425/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 1 3 8 6 - 1 4 2 5 ( 9 7 ) 0 0 0 5 0 - 4

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This wealth of information contrast with the poverty of information about N-substituted pyrazoles, even though the study of these compounds is essential to understand the effect of substituents through the nitrogen, a characteristic of aromatic azoles without counterpart in aromatic chemistry [7–9]. For the simplest of these N-substituted derivatives, 1-methylpyrazole 2, we are only aware of the work of Zerbi and Alberti [10] where the infrared spectra of the monoalkyl derivatives of pyrazole in the condensed-phase are given. Hence we have considered it of interest to measure the infrared spectrum of this species for the gas phase, as well as its Raman spectrum for the liquid phase. To complement this experimental information and to guide the assignment of the different fundamentals we have also carried out a molecular orbital study of this compound. 2. Experimental details The samples of N-methylpyrazole 2 used for our experiments were synthesized and purified using standard procedures [1]. The infrared spectrum for the vapour was recorded in a FTIR Nicolet 60 SX spectrometer, which incorporates a DGTS detector and a multipass cell adjusted to 12.70 m optical path. The data were obtained by coaddition of 50 scans at 0.5 cm − 1 resolution. The higher gas pressure of the sample in the spectra corresponded to the vapour pressure of N-methylpyrazole at room temperature (23°C). Two more spectra were run at lower pressures, attained by a partial evacuation of the cell. Background spectra were taken with the cell in position and in vacuum, before the sample was introduced and after it was completely evaculated. The Raman spectrum of the liquid was obtained by means of a Dilor XY spectrometer. The sample was introduced in a capillary tube and excited by the 514.5 nm line of a Ar + laser operating at 160 mW. 3. Computational details Electron correlation effects may be important when a reliable information on harmonic vibra-

tional frequencies is required. There are nowadays two kinds of methodologies which permit to include electron correlation effects in the calculations of harmonic vibrational frequencies, namely, ab initio calculations which introduced the correlation corrections by means of the Moller–Plesset perturbation theory at second order (MP2) [11] and density functional theory (DFT) approaches [12] which include appropriate correlation functions. It has been shown [13] that, in general, the DFT methods performed particularly well in reproducing harmonic vibrational frequencies. Actually, in general DFT values are closer to the experimental ones than those obtained at the ab initio MP2 level. In this work we have chosen as a suitable method the hybrid B3 functional proposed by Becke [14], which includes Slater, Hartree–Fock and Becke exchange functionals. The three parameters which give the contribution of these three terms are those proposed by Becke [14]. This hybrid B3 functional is combined with the gradient-corrected correlation functional of Lee, Yang and Parr (LYP) [15]. The structure of N-methylpyrazole was initially optimized at the HF/6-31G**. This geometry was then refined at the MP2(full)/6-31G** and B3LYP/ 6-31G** levels of theory. In all cases, the harmonic vibrational frequencies were evaluated by means of analytical second derivatives techniques. The corresponding Raman intensities and depolarization ratios were evaluated at the HF/6-31G** level exclusively. All these calculations have been carried out using the Gaussian-94 series of programs [16].

4. Results

4.1. Optimized geometry The numbering used for N-methylpyrazole 2 and the definition of the internal coordinates used to describe its normal vibrational modes are given in Fig. 1. The corresponding MP2/6-31G** and B3LYP/ 6-31G** optimized geometries are given in Table 1. This Table includes also the MP2/6-31G** and B3LYP/6-31G** optimized geometries of the unsubstituted parent compound taken from Ref.

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[17], for the sake of comparison. It is worth noting that both MP2 and B3LYP structures are rather similar. What is more important, the pyrazole ring is little affected by methyl substitution, although some changes are not negligible. In particular the C5N1N2 and the C4C5N1 bond angles which sizeably decrease and increase, respectively upon methyl substitution.

Table 1 Optimized geometries of N-methylpyrazole 2 and pyrazole 1 N-methylpyrazole 2

Pyrazole 1

MP2

MP2a

B3LYP

1.348

B3LYPa

N1 – N2

1.347

1.351

N2 – C3

1.348

1.334

1.347

1.333

C3 – C4

1.402

1.411

1.405

1.414

C4 – C5

1.384

1.382

1.385

1.381

1.350

C5 – N1

1.360

1.360

1.360

1.359

N1 – C6

1.447

1.450

1.007b

1.008b

C–H

1.077

1.082

1.077

1.082

C4 – H

1.077

1.079

1.076

1.079

1.080 1.091 1.094 1.091 112.3 104.6 111.9 104.2 106.8 119.5 128.4 131.7 128.1 109.2 111.0 109.2 166.3 46.8 8963.7

1.078

1.080

C5 – H 1.076 C – H10 1.085 C – H11 1.087 C6 – H12 1.085 C5N1N2 113.0 N1N2C3 104.0 N2C3C4 111.9 C3C4C5 104.7 C4C5N1 106.3 N2C3H 119.2 C3C4H 128.2 C4C5H 132.2 C5N1C6 127.9 N1C6H10 108.7 N1C6H11 110.4 N1C6H12 108.7 H10C6N1N2 166.5 H12C6N1N2 47.0 A 8977.3 (8970.107)c B 3730.1 (3745.544)c C 2680.0 (2682.667)c Ia 56.303 (56.340)c Ib 135.506 (134.927)c Ic 188.601 (188.386)c D 3.208 (2.881)c Fig. 1. (a) Numbering and internal coordinates definition for N-methylpyrazole 2. In plane coordinates: ri for Ci – H, R for N – CH3 and dij for C–C, C–N and N–N bond stretching coordinates. Angle bending: ai for CNN, NCC and CCC skeletal ring angles; bi for Ci –H bonds. Out of plane coordinates: gi for Ci –H bond bending and t(ring) for ring torsions. (b) Geometry of N-methylpyrazole and principal inertial axis.

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113.9 103.4 112.1 104.9 105.7 119.1 128.2 132.4 127.5

113.3 103.9 112.1 104.5 106.1 119.5 128.3 132.0 127.8

3715.9 2671.6 56.389 136.025 189.193 3.221

˚ , bond angles in degrees. Rotational constants, Bond lengths in A A, B, C are given in MHz, moments of inertia, Ia, Ib, Ic and the ˚ 2. The experimental inertial defect (D=Ia+Ib−Ic) are in amu. A values for these magnitudes are given within parenthesis. a Values taken from Ref. [17]; b N1 – H bond length; c Values taken from Ref. [18].

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These slight distortions of the pyrazole ring seem to be consistent with the experimental evidence [18]. Although, to the best of our knowledge, the experimental structure of N-methylpyrazole has not yet been resolved, its microwave spectrum was recorded by Krishnaji et al. [18]. In this work it was shown that the rotational constants of this system obtained assuming that the substitution of the methyl group does not change the structure of pyrazole, are slightly different from the experimental values. What is more important, the agreement between our calculated MP2/6-31G** rotational constants and the experimental ones is clearly better (see Table 1). This indicates that this optimized geometry, where the pyrazole ring is slightly distorted with respect to that of the parent compound, must be quite close to the real one. There is also a very good agreement between the calculated and the experimental moments of inertia. A similarly good agreement is also found for the inertial defect value (D) (see Table 1). It can be observed that this agreement is slightly better for MP2 than for DFT values. It is worth mentioning that both MP2 and B3LYP approaches predict the CS structure of N-methylpyrazole to be a transition state, with an imaginary frequency which corresponds to the methyl torsion. In both cases the methyl hydrogen closer to the pyrazole nitrogen lone-pair is predicted to be about 12° out of the plane of the molecule. This result is in contrast to that reported in Ref. [8], where one of the methyl hydrogen atoms lies in the plane of the molecule. The reason for this disagreement is that the structure reported in Ref. [8] was obtained starting from the HF optimized geometry, which has CS symmetry. As mentioned above, one of the harmonic vibrational frequencies of this conformer becomes imaginary when obtained at the MP2 level. It must be taken into account, however that the internal rotational barrier for this compound is rather low.

4.2. Internal rotational barrier We have also evaluated the potential energy curve corresponding to the internal rotational

barrier of the methyl group about the N1–C6 bond. The simplest approach is to consider both the pyrazole ring and the methyl group as rigid structures, so only the torsion angle (F) is allowed to vary. This would imply that neither the methyl group nor the azole system have time to relax their geometries along the rotation process. The second possibility would correspond to the situation where a relaxation is possible and it would imply to carry out a geometry optimization for each value of the torsion angle F. Both types of calculations have been performed at the MP2(full)/6-31G* level and the results have been plotted in Fig. 2. It should be noted that in the first case the potential presents two non-equivalent maxima at F: − 12° and F: 116°. This clearly reflects that in N-methylpyrazole the –CH3 group has no local C3n symmetry, since the three hydrogen atoms are not strictly equivalent. The two barriers are then slightly different, the lower being 0.560 kcal mol − 1.

Fig. 2. Potential energy curves for the internal rotation of N-methylpyrazole 2. Dashed line was obtained assuming that both the pyrazole ring and the methyl group behave as rigid structures. Full line was calculated by optimizing the geometry of the molecule for each value of the torsion angle F. F is the H12C6N1N2 dihedral angle. Values obtained at the MP2(full)/6-31G* level.

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Fig. 3. Infrared spectrum of N-methylpyrazole 2. The pressure was the vapor pressure of N-methylpyrazole at 23°C (approx. 5 torr). Optical path 12.70 m. Spectral resolution 0.5 cm − 1.

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Fig. 4. Additional infrared absorption regions of N-methylpyrazole 2 obtained in the experimental conditions indicated in Fig. 3.

When the geometry of the whole system is optimized for each value of F, F-0° and F= 120°, become equivalent situations and correspond to the maximum of the barrier (about 0.3 kcal mol − 1). It can be observed that this potential energy curve presents two local minima at F=47° and another one at F =73°. The only difference between the two situations is whether H10 is slightly above (H10C6N1N2= 167°) or slightly below (H10C6N1N2=193°) the plane defined by the pyrazole ring. It is worth noting that the rotational barrier predicted assuming that a geometry relaxation takes place along the rotational processes is sizeably smaller than the experimental estimations of Krishnaji et al. [18], which is closer to that obtained using a rigid model.

4.3. Infrared and Raman spectra Fig. 3 shows the infrared spectrum of Nmethylpyrazole in the gas phase recorded in the regions of C–H stretches (3200–2700 cm − 1) and of low frequency fundamentals (1800–550 cm − 1). Fig. 4 shows additional spectral regions in a more compressed format. Fig. 5 presents the Raman spectrum of the liquid, with the polarizatioin analyzer located parallel or perpendicular to the polarization direction of the incident beam. The wavenumbers of the centers of the main infrared bands are collected in Table 2 together with estimates of their intensity and band contour type. This Table contains also the Raman shifts of liquid N-methylpyrazole, together their measured intensities and depolarization ratios. The infrared bands reported by Zerbi and Alberti [10] for the

30 20 140

20

5 2 6

3014 2988 2943

2875 2818 2808

1516

1484

1444

1443

1520 1493

w

vs w

w w w w

Int.

1448.8 1446

1480

1742.5 1693.4 1642 1611.9 1559.8 1521.0

3149.7 3129.9 3122.3 3052 3036 3016.6 2991.1 2954 (2922.?) (2900.?) 2857 2840.1 (2829.9) 2820.5 2736.0 2729.4

Gas

C? B

B? w s

w w w w m vs

A B B A? A A

A? C? A

A A? b sh sh sh A?

m m s w m m, sh w, sh vs vw, sh w, sh vw m w m w w

Int.

A A/C? A A?

Type

n22 n8

n6 n7

n5

n21

n1 n2 n5 n4

a¦

a¦

Assignment

d(CH3)a¦ n (ring)

2n24 n24+n25 2n25 n24+n26 n25+n26 n (ring) d(CH3)a% 2n26

n(CH) n(CH) n(CH) n(CH3)a% 2n6 n(CH3)a¦ 2n7 n(CH3)s n7+n22 n7+n9 n22+n9 n6+n11 2n9 n6+n11 n12+n22 n6+n13, n10+ n11

5

2 1 50 1 1 3

835 755 683 654 612 373 219

12

26

967 919

7 3

64

1068 1032

1091

13

2 43 86

1367 1323 1278 1209

10 18

Int.

1418 1398

Liquid.

Raman

dp dp 0.26

dp

0.34

0.3 dp

0.25

0.22

dp? 0.18 0.19

dp? dp

Pol.

920 881 833? 755 683 650 not measured not measured not measured not measured

968

1066 1031

1090

1209

1323 1279

1397

Liquid.

Infrared

vs, b m m

m w

s

w m

s

m

vs

vs

Int.

968.5 943.6 920.8 873.0 821.1 741.3 687.8 652.3 610.1 not measured not measured not measured

1064.6 1028

1090.0

1423.6 1405.5 1388.6 1372 1325.3 1289.7 1261.5 1212.1 1178.3 1133.5

Gas

A/B C C C A C C

A

A A/B

A

B A/B A B? A A

A A A

Type

s w m s m vs s

m s

vs

m s vw m vw vw

m, sh vs m,sh

Int.

n18 n24 n25 n26 n19 n27 n28 n20 n29 n30

a¦ a¦

a¦ a¦

n (ring) n26+n29 d (ring) g(CH) g(CH) g(CH) d (ring) t (ring) t (ring) d(N–CH3) g(N–CH3) tCH3)

n17

d(CH) r(CH3)a% d(CH)

a¦ a¦ a¦

a¦

d(CH3)s n (ring) n26+n27 2n19 n (ring) n(N–CH3) n27+n28 d(CH) ? r(CH3)a¦

n15 n16

n14

n23

n13

n11 n12

n9 n10

Assigment

The nomenclature used is as follows: n denotes bond stretching, d denotes in plane bond bending and HCH angle deformation, g denotes out of plane bond bending, r denotes CH3 rocking and t denotes bond torsion. Raman intensities, measured as relative band heights, are given in arbitrary units. Estimations of polarization state are noted as p, for polarized, and dp, for depolarized; otherwise the ratio I /IÞ is given. Infrared intensities are reported as very strong (vs), strong (s), medium (m) and weak (w). (b) is used for broad and (sh) for shoulder.

p?

0.3

p? 0.18

dp dp? 0.18

1745 1709 1658 1587

170 sh? 94 sh

3137 3116 3108 3055

0.25 dp? 0.56 dp?

Liquid.

Int.

Liquid.

Pol.

Infrared

Raman

Table 2 Infrared and Raman spectra of N-methylpyrazole and their vibrational assignment (wavenumbers, cm−1)

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liquid have been also included for the sake of comparison. An ‘assignment’ column has been added according to the conclusions of forthcoming sections. Additional overtone and combination bands in the infrared spectrum of gaseous Nmethylpyrazole are listed and assigned in Table 3. In both tables estimations of the rotational contour of the infrared bands have been indicated.

4.4. Calculated 6ibrational data Table 4 presents the results obtained in our theoretical calculations for the harmonic vibrational frequencies, infrared and Raman intensities and depolarization ratios. The MP2 calculated frequencies have been scaled by the empirical factor 0.9426 proposed recently by Pople and coworkers [19] to account for the fact that MP2 values systematically overestimate the experimental ones. The comparison of calculated and experimental frequencies will be considered in detail in

forthcoming sections. In the assignment column, we give the dominant vibrational amplitudes of each normal mode. The Gaussian-94 programs calculate these amplitudes in terms of cartesian atomic displacement coordinates. Internal coordinates related to changes in bond lengths and bond angles are commonly used to describe the form of several normal modes. Following this use, the ‘assignment’ column, as given in Table 4, is a kind of a translation of a picture calculated in cartesian coordinates to a somewhat loose description in terms of internal vibrational coordinates. These coordinates may be chosen common to 1 and 2, as introduced in Fig. 1, plus the characteristic vibrations of the –CH3 group. The usual symbols for stretching, bending, etc. are specified in the footnote of Table 2. The bonds or angles which are involved in each particular vibration are given within parenthesis; the sign + or − simply indicate where the displacements are in-phase or out-of-phase.

4.5. Discussion

Fig. 5. Raman spectrum of N-methylpyrazole 2. The excitation line corresponds to the 514.5 mm line of an AR + laser. Spectral resolution 2.7 cm − 1.

The N-methylpyrazole molecule has 30 normal vibrational modes. Assuming that the pyrazole ring defines a symmetry plane, i.e. that the molecule belongs to the CS point group, 20 of these modes should be symmetric, a%, and 10 antisymmetric, a¦, respect to the reflection on the symmetry plane. For the atoms located in the plane of the molecule, the a% vibrational displacements take place in the plane of the molecule, the a¦ modes correspond to displacements out of the plane of the molecule. All vibrational modes are active in IR and Raman spectra. The 20 with a% symmetry can be polarized in Raman, the 10 a¦ should be depolarized. The infrared spectrum of the gas shows well developed band contours which depend on the direction of the vibrational dipolar moment respect to the principal axes of inertia. In Fig. 1(b) we present the molecular geometry in the frame of these principal axis. Type A, B or C bands correspond to (dm/dQ) being parallel respectively to the axis of the smaller (a), middle (b) or largest (c) moment of inertia. Both type A and type C bands show a neat PQR structure, the central Q branch being specially

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Table 3 Overtone and combination bands in the infrared spectrum of gaseous N-methylpyrazole (wavenumbers, cm−1) Obs.

Assign. (calc.)

Region 4500 – 4000 cm−1 a 4670 n1+n6 (4670) 4647 Q n2+n6 (4651) 4555 Q n1+n10 (4555) 4530 B? n2+n10 (4535) Region 2700 – 1800 cm−1 2694 n9+n12 (2713) 2659 A n13+n22 (2661) 2644 A 2n11 (2651) 2608 n11+n12 (2615) 2582 B? n6+n15 (2586) 2552 A n9+n23 (2557 C) 2491 A/B n10+n14 (2496) 2476? n16+n22 (2477 C) 2468 A n7+n17 (2461) 2439 A n6+n18 (2442) Á n +n 18 (2414) Í 7 2413 A Ä n11+n14 (2415)

Obs.

4446 4407 Q 4324 Q 4221 4210 B 2391 A 2352 A 2302 2277 2261 2254 2244 2210 2195 2154

A A A A A A C? B

Assign. (calc.)

Obs.

n5+n7 (4447) n5+n22 (4403)? n7+2n9 (4339) n2+n14 (4220) n3+n14 (4212)

4112 4068 4050 4042

Á n +n 18 (2399) Í 8 Ä n9+n17 (2392) Á n +n 15 (2354) Í 12 Ä n11+n16 (2354)

Assign. (calc.)

2121 A 2086 A

n13+n14 (2302) n13+n15 (2277) 2n23 (2267) n12+n17 (2258) n11+n18 (2246) n12+n18 (2211) n15+n23 (2198 C) n14+n15 (2155)

n1+n17 n1+n18 n2+n18 n3+n18

B A B A

Á 2n 15 (2129) Í Ä n14+n16 (2118) Á n +n 19 (2093) Í 10 Ä n15+n16 (2093)

n14+n17 (2058)

2061 B? 2040? 1999 A

(4118) (4071) (4051) (4043)

Á n +n 18 Í 14 Ä n16+n17

1972 A 1936 A 1890 A? 1780?

(2011) (1997)

2n17 (1937) n17+n18 (1889) n14+n19 (1778)

a

Large and diffuse (structureless) absorption bands appear between 4700 and 4100 cm−1, probably due to combinations involving n (CH3) vibrations. Only some band-like features are reported in the table.

prominent in type C bands. Type B bands have not central maximum. All these bands contours are seen in the IR spectrum of the N-methylpyrazole gas, as well as hybrid A/B bands in which (dm/dQ) makes an angle with the a and b axis. A, B or hybrid A/B bands correspond to vibrational modes with a% symmetry while type C bands characterize a¦ modes. If the molecule has no symmetry elements other than the identity, hybrid A/C and B/C bands may also occur. A set of 30 internal symmetry coordinates may be used to describe the vibrational normal modes. If we consider our molecule 2 as a pyrazole 1 derivative, we may choose 21 of these coordinates (15 a%+ 6 a¦) common to both molecules, six more as internal vibrational coordinates of the methyl group and the last three to describe the CH3 group twisting as a whole with respect to the pyrazole frame. With the aforementioned usual nomenclature, these internal coordinates may be enumerated as follows:

X-pyrazole coordinates

15 a%: 3n(CH)+ 5n(ring)+3d(CH) + 2d(ring)+n(N–X) + d(N–X)

(X= H or CH3)

6 a¦: 3g(CH)+ 2t(ring)+g(N–X)

–CH3

Internal

4 a%: 2n(CH3)+ 2d(CH3); 2 a¦: n(CH3)+d(CH3)

External

1 a%: r(CH3); 2 a¦: r(CH3)+t(CH3)

4.6. High-frequency 6ibrations In the infrared spectrum of gaseous Nmethylpyrazole, in the region of the aromatic C–H stretchings three PQR bands are observed,

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Table 4 Vibrational frequencies of N-methylpyrazole 2 n (cm−1)

Intensities

MP2

DFT

IR

Raman

3261 3149 3141 3083 3063 2968 1504 1484 1423 1450 1410 1415 1322 1267 1188 1110 1076 1055 1020 944 879 794 749 689 665 619 572 352 202 54

3284 3263 3250 3161 3131 3057 1571 1536 1492 1489 1463 1444 1365 1319 1249 1151 1116 1087 1055 988 933 887 829 753 698 665 621 372 207 57

1.0–1.9 2.3–3.1 3.0–6.4 5.5–8.9 11.3–15.8 32.8–44.6 15.0–17.8 9.4–4.5 7.9–7.2 7.9–7.0 15.3–6.7 5.5–33.9 3.8–1.0 14.7–17.7 1.2–1.9 3.1–3.2 7.0–15.4 12.8–11.3 10.8–9.11 7.0–15.8 3.0–4.0 6.2–3.2 11.4–1.5 47.4–47.8 9.0–8.1 5.9–5.2 1.8–3.8 4.0–4.0 5.4–5.3 0.5–0.5

124.4 30.3 102.1 58.7 83.5 141.1 0.0 11.6 4.3 18.3 9.1 38.6 8.0 9.8 2.3 2.2 9.3 3.5 2.9 7.5 5.5 1.4 0.5 0.9 7.6 0.3 0.7 0.2 3.1 0.2

(r)

Assignment

n(Exp.)

(0.1)

n1(a%) n(r3+r4+r5) n2(a%) n(r5−r4−r3) n3(a%) n(r3−r4+r5) n4(a%) n(CH3) n21(a¦) n(CH3) n5(a%) n(CH3)s n6(a%) n(d45−d15−d34) n7(a%) d(CH3) n8(a%) n(d15−d23−d45) n22(a¦) d(CH3)a¦ n9(a%) d(CH3)s n10(a%) n(d34−d23−d45)+d(CH) n11(a%) n(d12−d23−R) n12(a%) n23+d45−R) n13(a%) d(b3+b4+b5)+n(d23) n23(a¦) r(CH3)a¦ n14(a%) d(b3−b4−b5)+n(d34+R) n15(a%) d(b5−b4+b3)−r(CH3) n16(a%) d(b5−b4−b3)+r(CH3) n17(a%) d(b3−b4)+d(a1) n18(a%) d(a3−a4) n24(a¦) (g3−g4+g5) n25(a¦) (g5−g4−g3) n26(a¦) (g5+g4+g3) n19(a%) d(a1) n27(a¦) t (ring) n28(a¦) t (ring) n20(a%) d(N – CH3) n29(a¦) g(N – CH3) n30(a¦) t(CH3)

3149.7 3129.9 3122.3 3052 3016 2954 1521.0 1493a 1446 1449 1423.6 1405.5 1325.3 1289.7 1212.1 1133.5 1090.0 1064.6 1028.3 968.5 920.8 873.0 821.1 741.3 687.8 652.3 610.1 (373)b (219)b

(0.48)

(0.01)

(0.28) (0.15) (0.11) (0.21) (0.29) (0.41) (0.11) (0.16)

(0.33)

Values in cm−1. The MP2 values have been scaled by the empirical factor 0.9426. The Raman intensities were evaluated at the HF/6-31G** level. Depolarization ratios (r) are given only when different from 0.75. a IR liquid, b Raman liquid.

where the Q branch is well defined and with absorption maxima at 3149.7, 3129.9 and 3122.3 cm − 1, respectively. These absorptions are in good agreement as far as frequencies, intensities and polarization ratios are concerned, with the n1, n2 and n3 calculated ones (see Table 4). The three bands present A-type contours, although the Q branch of the intermediate absorption is broader than the other two. The corresponding bands for the liquid appear red shifted by 13 – 14 cm − 1 with respect to those in the gas phase. The same behaviour was found for pyrazole (and pyrazole1 d1), whose n(CH) bands in the gas phase appear

at 3154.3, 3136.5 and 3125.8 cm − 1, respectively. From the three n(CH3) bands expected for the methyl group, the ‘symmetrical’ one designated as n5 in Table 4, is predicted to be the strongest in the whole region in both IR and Raman spectra, and strongly polarized in the Raman spectrum. This points to the broad infrared band observed about 2954 cm − 1 in the gas phase which appears in the liquid (Raman) around 2943 cm − 1. This band present an atypical rotational contour, which together with its broadness, may be indicative of strong anharmonic couplings with low frequency torsional movements.

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Two n(CH3) bands are predicted of medium intensities and not polarized in the Raman spectrum. If the calculated values for n4 and n21 are scaled with the same ‘experimental factor’ found for n5, one gets 3086 or 3054 cm − 1 (MP2 or DFT) for n4 (a%) and 3049 or 3024 cm − 1 for n21 (a¦). Accordingly, the IR band observed in the gas at 3052 cm − 1 can be taken as n4 in spite of its very low Raman intensity. For the gas, the nearest observed band attributable to n21 would be that with a Q branch at 3016.6 cm − 1 in the IR, which is also Raman active and depolarized.

4.7. Low-frequency 6ibrations The theoretical calculations indicate that the highest frequency fundamental in this region, n6 [1504 (MP2) and 1571 (DFT) cm − 1], corresponds to the antisymmetric stretching of the double bonds of the pyrazole ring. The equivalent vibra-

Vibrational mode Calculated Frequencies

n%(MP2) n%(DFT) n Exp.

d(CH3)a% 1501 1487 1493 (liq.)

tion for pyrazole appears at 1531 cm − 1. In the infrared spectrum of gaseous N-methylpyrazole, there is a very strong A band at 1521 cm − 1 which fulfils the necessary requirements. Although the theoretical calculation predicts a null Raman intensity, a very weak polarized band can be observed at 1516 cm − 1 in the condensed phase. The next intense IR band in this region, observed at 1405.5 cm − 1 in the vapour, n10 is predicted to appear at 1410 (MP2) or 1444 (DFT) cm − 1 but with different assignments: at the MP2 level, it corresponds to the symmetric deformation (umbrella) of the methyl group, while at the DFT level, it would correspond to a symmetric stretching of the double bonds of the ring. Near these two vibrations, the calculations predict the existence of four other vibrations again with some ordering differences. That of higher frequency would correspond to the a% deformation of the methyl group. The assignment of the next three

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depends on the method. At the MP2 level the frequency of the a¦ deformation of the methyl group is higher than that of the ring vibration labelled n8 while at the DFT level the a¦ band is predicted to be 3 cm − 1 lower than n8. Experimentally, in this region of the gas-phase infrared spectrum, besides the two extreme bands at 1521 and 1405.5 cm − 1, already mentioned, three other bands can be observed: a B-type weak band centered around 1480 cm − 1, another one, a bit stronger, apparently also with type B contour, centered at 1416 cm − 1, showing a small Q branch (at 1448.8 cm − 1), and a third one at 1423.6 cm − 1, with an A or hybrid A/B type contour. The IR spectrum of the liquid shows an additional band at 1493 cm − 1 which in the spectrum of the vapour may be hidden by the much stronger 1521 cm − 1 band. If the calculated frequencies are re-scaled so to adjust n6 to the experimental value of 1521 cm − 1, the situation can be summarized as follows: d(CH3)a¦ 1466 1442 ?

n(ring) 1429 1444 1446

d(CH3)s 1426 1416 1423/1405

n(ring) 1431 1388

The B type band observed at 1480 cm − 1 is not included because it may well be taken as corresponding to 2n26, the overtone of the strongest infrared band in the whole spectrum of N-methylpyrazole. The question mark in the d(CH3)a¦ column points out that no clear type C band is seen in the IR spectrum in this region. The important issue remains the assignment of the 1423 and 1405 cm − 1 experimental bands. The DFT results predict the lowest frequency band to be the most intense both in infrared and Raman and to be depolarized, in agreement with the experimental observation, while according to the MP2 results the most intense band in the infrared does not coincide with the most intense in Raman. Furthermore, one should expect the g(CH3)s transition to be polarized in Raman, which may be the case only if this vibration is assigned to the IR band of the gas

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observed at 1423.6 cm − 1 and at 1418 cm − 1 for the liquid in Raman, in agreement with the DFT results. There are two other arguments which seem to ratify this assignment. On the one hand, the first overtone of the g(CH3)s mode, to be discussed later, is found in infrared around 2830 cm − 1, value consistent with a frequency for the g(CH3)s mode of 1423 cm − 1 but not of 1405.5 cm − 1. On the other hand, the 1423 cm − 1 value is in very good agreement with that proposed by Perchard and Novak [20] for this mode in N-methylimidazole (1421 cm − 1) and by Scott [21] for N-methylpyrrole (1416 cm − 1). If the DFT values are scaled again, adjusting now the calculated n10 to the observed value of 1405 cm − 1, a good agreement is found between calculated and experimental wavenumbers: Vibrational mode n¦(DFT) n Exp.

d(CH3)a% 1495* 1493*

d(CH3)a¦ 1449.3 1449 (?) 1442

As previously said, no experimental type C contour band can be seen assignable to d(CH3)a¦. Looking for a possible vestige, we propose the small peak (a Q branch?) at 1448.8 cm − 1 or a sudden profile change observed at 1442 cm − 1. It should be noted that the calculation of a% and a¦ vibrational modes are in fact two separable problems, due to symmetry, and a strict close ordering is not necessarily found between them. At lower frequencies, the bands observed in the gas-phase at 1325, 1290 and 1212 cm − 1, can be easily correlated with the calculated vibrational modes labelled n11, n12 and n13, respectively. The second one, n12, described as the N –CH3 stretching is rather peculiar, in the sense that, superposed to an hybrid A/B type contour appear a sequence of regularly separated lines (about 1.2 cm − 1), as it is clearly illustrated in Fig. 6, where this region has been significantly enlarged. This structure is probably associated with a coupling of this vibrational mode with torsional or free rotation features of the methyl group. On the other side, calculated rotational

contours of type B bands show semi-resolved structures similar to those observed on the P and Q branches of n11. The n23 vibration, which corresponds to the a¦ rocking of the methyl group is assigned to the small peak at 1133.5 cm − 1 located at higher frequency than its a% counterpart, n15, which is assigned to the band observed at 1064.6 cm − 1. The three bands assigned to n14, n16 and n17, correlate reasonably well with the calculates ones, even when their intensities are concerned. They correspond to ring stretchings and CH bendings, which appear often coupled, like in NH-pyrazole. The bands observed in the gas-phase infrared spectrum at 920.8 and 687.8 cm − 1, respectively n(ring) 1452.2 1446

d(CH3)s 1424 1423.6

n(ring) [1405.5] 1405.5 1405.5

correspond necessarily to the in-plane deformations of the pyrazole ring. Both are bands with an A or a hybrid A/B (but not C) contour and their assignment can be made without ambiguity, since there are no other bands to mix with in this region.

Fig. 6. Structure on the band contour of the n12, n(N – CH3), infrared band.

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In the 900 – 550 cm − 1 range five bands with C contour are clearly observed. The first three can be assigned to C – H out-of-plane bendings (n24, n23 and n26) and the last two (n27 and n28) to ring torsions. In the condensed phase these bands, in particular n25 by the MP2 calculations is very different from that obtained at the DFT level. Again, the experimental observations agree better with the DFT results, since the n25 transition is about half less intense than n24. Two additional bands at lower frequencies are still visible in the Raman spectrum of the liquid. They correspond to n20(a%) and n29(a¦), which are the in-plane and the out-of-plane N – CH3 bendings.

between the symmetrical n(CH3) vibration and the 2d(CH3) vibrations. Such a resonance is found also here with 2n9, the overtone of the symmetrical CH3 bending vibration, giving rise to the IR absorption features around 2830 cm − 1 and to the polarized Raman bands in the liquid around 2818 cm − 1. Besides 2n9, other near lying combination bands as specified in Table 3, contribute to the complex IR absorption profile around 2830 cm − 1. Other combination bands involving the three d(CH3) vibrations are overlapped by the much stronger n5, n(CH3)s, fundamental vibration (see Table 3).

4.8. O6ertones and combination bands

As an aid to visualize the effects of N-substitution on the vibrational spectra of pyrazole, Fig. 7 presents a correlation of the fundamental frequencies (below 1600 cm − 1) of pyrazole and Nmethylpyrazole via pyrazole-1d1. The labels over the pyrazole frequency lines describe approximately the corresponding vibrational modes according the assignments of Majoube [5]. The correlation with the pyrazole-1d1 modes also follows these assignments. The corresponding labels under the N-methylpyrazole frequencies come from our calculations, as given in Table 4. Durig et al. [6], who quote the same wavenumbers as Majoube for the a% vibrational modes, find however an assignment different in several points Moreover these authors locate the higher g(CH) a¦ mode at a different frequency (1034 cm − 1 instead of 878.8 cm − 1). Our results for N-methylpyrazole comply better with the description of Majoube for both symmetry species a% and a¦. It is recognized in both papers that d(NH) and d(CH) vibrational coordinates contribute significatively to several vibrational modes, mixing with ring stretching and ring bending coordinates. Majoube [5] also notes that, going from pyrazole to pyrazole-1d1, the perturbation induced by N-deuteration introduces large changes, frequency shifts as well as intensity changes. In the diagram of Fig. 7 we have specially considered the region between 1300 and 1100 cm − 1 in which a strong coupling seems to

Most of the observed bands which have not been assigned as fundamentals may be explained as overtone or combination bands as indicated in Tables 3 and 4. Only a few comments will be added here. The features observed in the region 4500–400 cm − 1 are due to combinations of the n(CH) or n(CH3) fundamentals with other vibrations, among them the d(CH3) fundamentals. A diffuse, structureless absorption seems to be associated with the n(CH3)s +d(CH3) combinations. It is also remarkable that in combinations of the n(CH) fundamentals, n1 to n3, with other fundamentals, those of n1 and n3 present contour types different from those of n2 (Ex., in Fig. 4, the three bands around 4050 cm − 1). Around the high frequency n(CH3) vibrations (3200–2700 cm − 1) overtones and combination bands characteristic of the – CH3 group are observed. At frequencies higher than that of n5 one should look for the first overtones of n6 (a ring stretching vibration) and n7 (nonsymmetric d(CH3), a%) expected to be about 3040 and 3000 cm − 1, respectively. There is probably a type B band with an absorption minimum at 3036 cm − 1 corresponding to 2n6. The small peak at 2991 cm − 1 could correspond to 2n7. In this case 2n7 would not be affected by the vicinity of n4 a situation opposite to that found in many molecules which show strong Fermi resonances

4.9. Correlation with the pyrazole spectra

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Fig. 7. Correlation diagram.

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occur in pyrazole between three vibrational coordinates: d(NH), d(CH) and n(ring) (approximately a ring breathing vibration). By N-deuteration the frequency of the d(ND) vibration is 1.4 times lower than that of the d(NH) and the triple mixing is thus decoupled, so that in the pyrazole-1d1 remain three separate characteristic vibrations: d(CH) (1210 cm − 1), n(ring) (1136 cm − 1) and d(ND) (858 cm − 1). Note that this interpretation assumes that an uncoupled d(NH) in pyrazole falls around 1210 cm − 1; in the Durig et al. approximate assignments [6] d(NH) is supposed to be around 1410 cm − 1). Only two additional couplings seem to be present in the case of N-methylpyrazole according our assignments (Table 4), one around 1300 cm − 1 involving the n(N – CH3) and a n(ring) vibration, another around 1050 cm − 1 mixing the a% r(CH3) and a n(ring) vibration. It is interesting to note that all other vibrational modes of the –CH3 group seem not to mix appreciably with those of the pyrazole structure. We want also remind in this section the bands quoted by Zerbi and Alberti [10] as characteristic of pyrazole in N-substituted alkyl pyrazoles, in the IR spectra of liquid samples. From a small series of four derivatives (going from methyl to butyl derivatives), they note the following as characteristic of N-substitution: 1520, 1397, 1279 cm − 1, a doublet from 1100 to 1080 cm − 1 and a very strong band at 755 cm − 1. Besides, the presence of a pyrazole monosubstituted ring may be indicated by strong bands at about 1050 and 940 cm − 1 and a weak doublet at 675 and 650 cm − 1. All these bands may find approximate descriptions with the aid of the correlations shown in Fig. 7.

5. General conclusions The gas-phase infrared spectrum of Nmethylpyrazole 2 was measured in the range 5000500 cm − 1 and its Raman spectrum was obtained in the condensed-phase in the range 3500–100 cm − 1. The corresponding depolarization ratios were also measured. We have found a fairly good correlation between the experimental vibrational frequencies and the calculated harmonic ones.

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This agreement is similarly good when the latter are obtained either at the MP2/6-31G** or the B3LYP/6-31G** levels of theory. This theoretical information was useful in the assignment of the different fundamentals. We have found however some significant disagreements as far as the infrared and Raman intensities of some transitions are concerned. Compound 2 was selected because the methyl group was expected to slightly modify the structure of the pyrazole ring and because this compound, being a liquid at room temperature, was expected to make the measurements in the gasphase easier. Both expectations were confirmed. Moreover, the present work confirms the reliability of high-level theoretical calculations for assigning vibrational spectra and, reciprocally, the use of a good quality IR and Raman spectra to verify the quality of the calculations. These facts being ascertained, it is now possible to undertake more complex problems like those present in other 1substituted pyrazoles, for instance, 1-aminopyrazole.

Acknowledgements This work has been partially supported by the DGICYT Project No. PB93-0287-C02-01. J.M.O. acknowledges also partial support by the Project COR 031/94 from the Comunidad de Madrid. We would like to thank Dr Marı´a V. Garcı´a Pe´rez, responsible of the Servicio de Espectroscopı´a Raman del Departamento de Quı´mica Fı´sica de la Facultad de Quı´mica de la Universidad Complutense de Madrid, who recorded the Raman spectra. References [1] J. Elguero, Pyrazoles and their benzo derivatives, in: A.R. Katritzky and C.W. Rees (Eds.), Comprehensive Heterocyclic Chemistry, Vol. 5, Pergamon Press, Oxford, 1984; J. Elguero, Pyrazoles and their benzo derivatives, in: A.R. Katritzky, C.W. Rees and E.F.V. Scriven (Eds.), Comprehensive Heterocyclic Chemistry II, Vol. 3, Pergamon Press, New York, 1996. [2] V. Tabacik, V. Pellegrin, Hs.H. Gu¨nthard, Spectrochim. Acta A35 (1979) 1055.

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[3] V. Tabacik, Hs.H. Gu¨nthard, J. Mol. Spectrosc. 45 (1973); V. Tabacik, S. Sportouch, J. Raman Spectrosc. 7 (1978) 61. [4] M. Majoube, J. Phys. Chem. 92 (1988) 2407. [5] M. Majoube, J. Raman Spectrosc. 20 (1989) 49. [6] J.R. Durig, M. Mamula Bergana, V.M. Zunic, J. Raman Spectrosc. 23 (1992) 357. [7] A.L. Llamas-Saiz, C. Foces-Foces, J. Elguero, J. Mol. Struct. 319 (1994) 231. [8] O. Mo´, M. Ya´n˜ez, A.L. Llamas-Saiz, C. Foces-Foces, J. Elguero, Tetrahedron 51 (1995) 7045. [9] R.M. Claramunt, D. Sanz, J.A. Jime´nez, M.L. Jimeno and J. Elguero, Heterocycles, (1997) in press. [10] G. Zerbi, C. Alberti, Spectrochim. Acta 18 (1962) 407. [11] W.J. Hehre, L. Radom, P.V.R. Schleyer, J.A. Pople, Ab Initio Molecular Orbital Theory, Wiley, New York, 1986. [12] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, Oxford, 1989. [13] C.W. Bauschlicher Jr., Chem. Phys. Lett. 246 (1995) 40. [14] A.D. Becke, J. Chem. Phys. 98 (1993) 5648; ibid. 96 (1992) 2155.

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