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Vibrational spectrum of methyl pyrrole-2-carboxylate
Spectrochimica Acta Part A 58 (2002) 213– 215 www.elsevier.com/locate/saa
Letter
Vibrational spectrum of methyl pyrrole-2-carboxylate Alina T. Dubis, Sławomir J. Grabowski * Institute of Chemistry, Uni6ersity of Białystok, 15 -443 Białystok, Al.J. Piłsudskiego 11, Poland Received 8 February 2001; accepted 3 May 2001
Keywords: Conformational IR; Methyl pyrrole-2-carboxylate; Ab initio calculations
tional frequencies for the fully optimized structures were considered in this paper.
1. Introduction The conformational IR and theoretical analysis of methyl pyrrole-2-carboxylate has been presented in our earlier study [1]. The aim of this work is the application of building parts spectral (FT-IR) analysis and ab initio calculations of methyl pyrrole-2carboxylate, i.e. pyrrole and methyl formate to spectral assignment of this ester. In case of composed organic structures there are many bands that are difficult to interpret [2]. These difficulties may be avoided by detailed analysis of IR spectra of building compounds. Ab initio calculations may be also useful to avoid the difficulties connected with the bands interpretation. In this study the vibrational spectra of the model compounds were recorded as CCl4 diluted solutions and they were compared with the theoretically predicted spectra obtained from ab initio calculations. The geometry of methyl pyrrole-2-carboxylate (MPC), pyrrole (PYR) and methyl formate (MF) has been optimized at RHF/6-311 + + G** level of theory using the Gaussian-94 program [3]. The theoretical vibra* Corresponding author. Fax: + 48-85-745-7581. E-mail addresses: [email protected] (A.T. [email protected] (S.J. Grabowski).
Dubis),
2. Results and discussion Pyrrole and methyl formate molecules analyzed in this study represent the building parts of methyl pyrrole-2-carboxylate molecule. Most of the absorption bands in the spectrum of MPC were also found in spectra of PYR and MF. Close inspection of the MPC spectrum shows that some of the absorption bands are shifted in comparison with Table 1 Experimental and theoretical frequencies of N–H, CO and CH groups in MPC, PYR and MF Compounda
wNH
Ring wCH
Experimental frequencies (cm−1) MPC 3465 3098, 3070, 2994 PYR 3495 3120, 3107, 3040 MF
wCO
1701 1735
−1
Theoretical frequencies (cm MPC 3474 PYR 3495 MF
) 3049, 3042, 3025 3048, 3043, 3027
1724 1760
a MPC, methyl pyrrole-2-carboxylate; PYR, pyrrole; MF, methyl formate.
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 5 1 1 - X
214
A.T. Dubis, S.J. Grabowski / Spectrochimica Acta Part A 58 (2002) 213–215
Table 2 Selected geometrical parameters of MPC, MF and PYR molecules obtained within HF/6-311++G** level of theory
Bond lengths in A, , angles in °. All designations in accordance with schemes 1–3.
corresponding bands of PYR and MF. The comparison of the spectra of MPC and MF shows that the influence of a pyrrole aromatic ring on the vibration frequency of carbonyl group is significant.
A.T. Dubis, S.J. Grabowski / Spectrochimica Acta Part A 58 (2002) 213–215
The carbonyl group conjugation with the aromatic p-electron system leads to reducing of double bond character and it implies lower absorption frequency with 34 cm − 1 in MPC in comparison with MF (Table 1). Furthermore, the characteristic N– H stretching frequencies for MPC are lower with 30 cm − 1 than the respective frequency of PYR and the pyrrole ring C–H stretching frequencies are also lower with 22, 37 and 46 cm − 1 for MPC than for pyrrole. The theoretically predicted differences in wNH and wCO modes (21 and 36 cm − 1) are close to their experimental values (Table 1). In case of the ring wCH frequencies only 1–2 cm − 1 differences are observed. The agreement between measured and scaled theoretical frequencies is very good.
215
of the AIM topological parameters [5,6] shows that there is no intramolecular H-bond for MPC [1]. Hence the elongation of N– H and CO bonds for MPC is probably connected with the conjunction of the carbonyl group with the aromatic p-electron system. Table 2 also shows that the symmetry of pyrrole aromatic ring is broken for the MPC molecule due to the effect of the ester substituent. Analysis of the experimental FT-IR and theoretically calculated spectra leads to the assignment of the vibrational bands. Ab initio calculations provide good estimates for the vibrational frequencies of analyzed molecules. The fundamental frequencies of methyl pyrrole-2-carboxylate are easier to assign using the assignments of pyrrole and methyl formate. The geometrical differences between MPC, PYR and MF molecules are in agreement with the IR analysis.
3. The geometry of MPC, PYR and MF molecules The full geometry optimisations were performed for MPC, PYR and MF molecules at RHF/6-311 + + G** level of theory. Table 2 presents the selected geometrical parameters for these molecules. The results are in agreement with the theoretical and experimental IR spectra presented in Table 1. If we compare the lengths of N– H and CO bonds of PYR and MF molecules respectively with the corresponding bonds of MPC then their elongation is observed. It could be caused by intramolecular H-bonding existence within the MPC molecule. Such elongation due to H-bonding interaction was detected for the experimentally investigated O– H···OC systems [4]. However it was pointed out in the previous paper [1] that for the MPC molecule the H-bonding existence can only hardly be accepted if one takes into account the geometrical criteria. The analysis
References [1] A.T. Dubis, S.J. Grabowski, J. Mol. Struct. 562 (2001) 107. [2] J. Cybulski, Z. Chilmonczyk, M. Glice, M. Cybulski, K. Bajdor, A. Les, J. Mol. Struct. 404 (1997) 221. [3] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.A. Cheeseman, T.A. Keith, A. Peterson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, L.R. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Barker, J.P. Stewart, M. HeadGordon, C. Gonzales, J.A. Pople, Gaussian-94 (revision E.3), Gaussian Inc., Pittsburgh, PA, 1995. [4] S.J. Grabowski, Tetrahedron 54 (1998) 10153. [5] R.F.W. Bader, Atoms in Molecules, A Quantum Theory, Oxford Science Publications, Clarendon Press, London, 1990. [6] F.W. Klieger-Konig, R.W.F. Bader, T.H. Tang, J. Comput. Chem. 3 (1982) 317.
Letter
Vibrational spectrum of methyl pyrrole-2-carboxylate Alina T. Dubis, Sławomir J. Grabowski * Institute of Chemistry, Uni6ersity of Białystok, 15 -443 Białystok, Al.J. Piłsudskiego 11, Poland Received 8 February 2001; accepted 3 May 2001
Keywords: Conformational IR; Methyl pyrrole-2-carboxylate; Ab initio calculations
tional frequencies for the fully optimized structures were considered in this paper.
1. Introduction The conformational IR and theoretical analysis of methyl pyrrole-2-carboxylate has been presented in our earlier study [1]. The aim of this work is the application of building parts spectral (FT-IR) analysis and ab initio calculations of methyl pyrrole-2carboxylate, i.e. pyrrole and methyl formate to spectral assignment of this ester. In case of composed organic structures there are many bands that are difficult to interpret [2]. These difficulties may be avoided by detailed analysis of IR spectra of building compounds. Ab initio calculations may be also useful to avoid the difficulties connected with the bands interpretation. In this study the vibrational spectra of the model compounds were recorded as CCl4 diluted solutions and they were compared with the theoretically predicted spectra obtained from ab initio calculations. The geometry of methyl pyrrole-2-carboxylate (MPC), pyrrole (PYR) and methyl formate (MF) has been optimized at RHF/6-311 + + G** level of theory using the Gaussian-94 program [3]. The theoretical vibra* Corresponding author. Fax: + 48-85-745-7581. E-mail addresses: [email protected] (A.T. [email protected] (S.J. Grabowski).
Dubis),
2. Results and discussion Pyrrole and methyl formate molecules analyzed in this study represent the building parts of methyl pyrrole-2-carboxylate molecule. Most of the absorption bands in the spectrum of MPC were also found in spectra of PYR and MF. Close inspection of the MPC spectrum shows that some of the absorption bands are shifted in comparison with Table 1 Experimental and theoretical frequencies of N–H, CO and CH groups in MPC, PYR and MF Compounda
wNH
Ring wCH
Experimental frequencies (cm−1) MPC 3465 3098, 3070, 2994 PYR 3495 3120, 3107, 3040 MF
wCO
1701 1735
−1
Theoretical frequencies (cm MPC 3474 PYR 3495 MF
) 3049, 3042, 3025 3048, 3043, 3027
1724 1760
a MPC, methyl pyrrole-2-carboxylate; PYR, pyrrole; MF, methyl formate.
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 5 1 1 - X
214
A.T. Dubis, S.J. Grabowski / Spectrochimica Acta Part A 58 (2002) 213–215
Table 2 Selected geometrical parameters of MPC, MF and PYR molecules obtained within HF/6-311++G** level of theory
Bond lengths in A, , angles in °. All designations in accordance with schemes 1–3.
corresponding bands of PYR and MF. The comparison of the spectra of MPC and MF shows that the influence of a pyrrole aromatic ring on the vibration frequency of carbonyl group is significant.
A.T. Dubis, S.J. Grabowski / Spectrochimica Acta Part A 58 (2002) 213–215
The carbonyl group conjugation with the aromatic p-electron system leads to reducing of double bond character and it implies lower absorption frequency with 34 cm − 1 in MPC in comparison with MF (Table 1). Furthermore, the characteristic N– H stretching frequencies for MPC are lower with 30 cm − 1 than the respective frequency of PYR and the pyrrole ring C–H stretching frequencies are also lower with 22, 37 and 46 cm − 1 for MPC than for pyrrole. The theoretically predicted differences in wNH and wCO modes (21 and 36 cm − 1) are close to their experimental values (Table 1). In case of the ring wCH frequencies only 1–2 cm − 1 differences are observed. The agreement between measured and scaled theoretical frequencies is very good.
215
of the AIM topological parameters [5,6] shows that there is no intramolecular H-bond for MPC [1]. Hence the elongation of N– H and CO bonds for MPC is probably connected with the conjunction of the carbonyl group with the aromatic p-electron system. Table 2 also shows that the symmetry of pyrrole aromatic ring is broken for the MPC molecule due to the effect of the ester substituent. Analysis of the experimental FT-IR and theoretically calculated spectra leads to the assignment of the vibrational bands. Ab initio calculations provide good estimates for the vibrational frequencies of analyzed molecules. The fundamental frequencies of methyl pyrrole-2-carboxylate are easier to assign using the assignments of pyrrole and methyl formate. The geometrical differences between MPC, PYR and MF molecules are in agreement with the IR analysis.
3. The geometry of MPC, PYR and MF molecules The full geometry optimisations were performed for MPC, PYR and MF molecules at RHF/6-311 + + G** level of theory. Table 2 presents the selected geometrical parameters for these molecules. The results are in agreement with the theoretical and experimental IR spectra presented in Table 1. If we compare the lengths of N– H and CO bonds of PYR and MF molecules respectively with the corresponding bonds of MPC then their elongation is observed. It could be caused by intramolecular H-bonding existence within the MPC molecule. Such elongation due to H-bonding interaction was detected for the experimentally investigated O– H···OC systems [4]. However it was pointed out in the previous paper [1] that for the MPC molecule the H-bonding existence can only hardly be accepted if one takes into account the geometrical criteria. The analysis
References [1] A.T. Dubis, S.J. Grabowski, J. Mol. Struct. 562 (2001) 107. [2] J. Cybulski, Z. Chilmonczyk, M. Glice, M. Cybulski, K. Bajdor, A. Les, J. Mol. Struct. 404 (1997) 221. [3] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.A. Cheeseman, T.A. Keith, A. Peterson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, L.R. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Barker, J.P. Stewart, M. HeadGordon, C. Gonzales, J.A. Pople, Gaussian-94 (revision E.3), Gaussian Inc., Pittsburgh, PA, 1995. [4] S.J. Grabowski, Tetrahedron 54 (1998) 10153. [5] R.F.W. Bader, Atoms in Molecules, A Quantum Theory, Oxford Science Publications, Clarendon Press, London, 1990. [6] F.W. Klieger-Konig, R.W.F. Bader, T.H. Tang, J. Comput. Chem. 3 (1982) 317.