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Vibrational spectra, structure, and theoretical calculations of 2-fluoro- and 3-fluoropyridine
Spectrochimica Acta Part A 79 (2011) 1191–1195
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Vibrational spectra, structure, and theoretical calculations of 2-fluoro- and 3-fluoropyridine Praveenkumar Boopalachandran, Jaan Laane ∗ Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, United States
a r t i c l e
i n f o
Article history: Received 3 March 2011 Received in revised form 23 March 2011 Accepted 15 April 2011 Keywords: Infrared Raman Ab initio DFT Fluoropyridine Structure
a b s t r a c t The infrared and Raman spectra of liquid and vapor-phase 2-fluoropyridine and 3-fluoropyridine have been recorded and assigned. Ab initio and DFT calculations were carried out to compute the molecular structures and to verify the vibrational assignments. The observed and calculated spectra agree extremely well. The ring bond distances of the fluoropyridines are very similar to those of pyridine except for a shortening of the C–N(F) bond in 2-fluoropyridine. The C–F bond stretching frequencies are similar to that in fluorobenzene reflecting the influence of the ring bonding. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In 2008 we reported the ultraviolet absorption spectrum of pyridine and showed that the molecule is very floppy and quasiplanar in its S1 (n,*) electronic excited state [1]. Determination of the potential energy function for the ring-bending vibration showed the barrier to planarity to be only 3 cm−1 . This is in contrast to the S0 ground state where pyridine is rigidly planar. In a continuation of our work on the vibrations and structure of pyridine and substituted pyridines in their ground and excited states, in the work described here we present our results on two substituted fluoropyridines in their electronic ground state. The infrared and Raman spectra of 2-fluoropyridine (2FPy) and 3fluoropyridine (3FPy) have been analysed and their vibrational frequencies have been determined. Ab initio and DFT computations were carried out to compute the structures of these molecules and to complement the experimental work and confirm the vibrational assignments. Green and co-workers [2] have previously reported the infrared and Raman spectra and presented partial assignments for these molecules for the electronic ground state, but no structural information has been previously reported.
2-Fluoropyridine (2FPy) and 3-fluoropyridine (3FPy) (99% purity) were purchased from Aldrich and purified by trap to trap distillation. The Raman spectra of the molecules in the vapor-phase were recorded of samples sealed in specially designed glass cells which have previously been described [3]. The vapor pressures of the samples at room temperature were about 9 Torr for 2FPy and 15 Torr for 3FPy. A Jobin-Yvon U-1000 spectrometer equipped with a liquid nitrogen-cooled CCD detector was used to collect the spectra. The 532 nm line of a frequency-doubled Nd:YAG Coherent Verdi-10 laser was used and typically operated at 5 W of power. Spectral scans spanning 60 cm−1 were typically recorded over periods of 4–6 h so that many hundreds of individual spectra could be averaged. The spectral resolution was 0.7 cm−1 . The liquid phase Raman spectra were also collected on the same instrument with samples in glass cuvettes using a laser power of 500 mW. The liquid-phase and vapor-phase mid-infrared spectra of 2FPy and 3FPy were collected on a Bruker Vertex 70 FT spectrometer equipped with a globar light source, a KBr beamsplitter and deuterated lanthanum triglycine sulfate (DLaTGS) detector. The vapor-phase far infrared spectra (60–600 cm−1 ) were also collected on the same instrument equipped with a mylar beamsplitter, and a mercury cadmium telluride (MCT) detector. The vapor pressures of the samples were the same as for the Raman measurements. Typically 1024 scans were collected using a resolution of 0.5 cm−1 .
∗ Corresponding author. E-mail address: [email protected] (J. Laane). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.04.041
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P. Boopalachandran, J. Laane / Spectrochimica Acta Part A 79 (2011) 1191–1195
Fig. 1. Calculated structures of (a) pyridine, (b) 2-fluoropyridine, and (c) 3-fluoropyridine.
x4
N
F
Transmittance
calculated
x4
liquid
x4
vapor 3000
1600
cm-1
1200
800
Fig. 2. Liquid, vapor, and calculated infrared spectra of 2-fluoropyridine.
calculated
Raman Intensity
N
liquid
x4
vapor
3200
2800
1600
1200
cm-1
800
Fig. 3. Liquid, vapor and calculated Raman spectra of 2-fluoropyridine.
400
F
P. Boopalachandran, J. Laane / Spectrochimica Acta Part A 79 (2011) 1191–1195
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Table 1 Observed and calculated vibrational frequencies (cm−1 ) for 2-fluoropyridine. Cs
Approximate description
A (i.p.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
C–H stretch C–H stretch C–H stretch C–H stretch Ring stretch Ring stretch Ring stretch Ring stretch C–H wag Ring stretch C–F stretch C–H wag C–H wag C–H wag Ring breathing Ring bend Ring bend Ring bend C–F wag C–H wag C–H wag C–H wag C–H wag Ring twist Ring bend Ring bend C–F wag
Ramana
Infrared Liquid
A (o.p.)
3100 sh 3100 sh 3086 3069 1598 1581 1473 1436 1301 1295 1258 br 1143 1098 1045 994 840 621 554 433 983 963 873 780 733 519 417 –
Vapor w w mw mw vs vs vs vs m m vs s m m s vs ms s w w w w vs m m m –
3100.1 3092.4 3080.9 – 1604.5 1593.0 1477.6 1438.7 1302.9 1286.3 1265.9 1139.4 1097.9 1044 sh 996.6 842.3 620.1 553.8 432.3 – 960.5 868.2 780.4 732.5 517.7 413.8 –
Liquid s s s – s vs vvs vvs m m vs vs mw m mw vs w m vw – mw mw ms m m mw –
3101 br 3101 br 3084 3071 1596 1579 1471 1434 1301 – 1259 br 1141 1096 1044 993 839 620 553 431 982 962 870 783 733 517 416 228
Vapor (6) (6) (14) (26) (5) (4) (1) (0.5) (6) – (6) (2) (8) (41) (100) (25) (11) (31) (3) (3) (0.1) (0.5) (0.6) (1) (2) (0.6) (29)
3100 br 3092 3080 3077 – – 1478 1439 1303 1286 1265 1139 1098 1045 999 842 620 554 433 982 963 869 – – – – 226
(9) (42) (56) (60) – – (2) (2) (18) (7) (22) (2) (7) (182) (100) (125) (6) (40) (7) (2) (1) (1) – – – – (20)
Calculatedb
GKPc
Intensity
3096 3086 3066 3050 1610 1597 1480 1441 1306 1289 1247 1148 1100 1046 995 834 625 551 427 979 962 871 781 732 517 419 216
(0.7, 100) (10, 70) (6, 44) (10, 55) (61, 43) (79, 43) (78, 5) (79, 2) (3, 19) (2, 14) (140, 62) (6, 10) (3, 24) (7, 71) (6, 100) (36, 62) (2, 19) (6, 24) (0.6, 0.1) (0.1, 0.4) (1, 0.1) (1, 0.1) (70, 1) (6, 1) (4, 2) (4, 0.4) (0, 10)
3097 3094 3074 3074 1598 1580 – – 1303 – 1249 1146 1099 1045 996 828 622 556 – – – – – – – – 230
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; i.p., in-plane; o.p., out-of-plane. a Relative intensities in parenthesis. b B3LYP/6-311++g(d,p); frequencies scaled with a scaling factor of 0.985 for frequencies less than 1800 cm−1 and 0.964 for frequencies greater than 1800 cm−1 . The calculated relative intensities are shown as (IR, Raman). c Ref. [2].
Table 2 Observed and calculated vibrational frequencies (cm−1 ) for 3-fluoropyridine. Cs
Approximate description
Infrared
A (i.p.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
C–H stretch C–H stretch C–H stretch C–H stretch Ring stretch Ring stretch Ring stretch Ring stretch C–H wag Ring stretch C–F stretch C–H wag C–H wag C–H wag Ring breathing Ring bend Ring bend Ring bend C–F wag C–H wag C–H wag C–H wag C–H wag Ring twist Ring bend Ring bend C–F wag
– 3070 – 3049 1594 1583 1478 1427 1319 1248 1227 1189 1098 1038 sh 1024 816 sh 615 535 – 981 938 905 804 702 497 415 –
Ramana
Liquid
A (o.p.)
Vapor – m – m m s vs vs w vs vs m ms w ms ms mw s – vw w w vs s w mw –
– 3076.5 – 3054.1 – 1588.4 1480.1 1425.8 1315.6 1249.4 1227.4 1187.1 1096.0 – 1021.8 816.4 – 533.3 – 974.1 934.0 905.1 800.7 701.0 506.8 411.7 –
Liquid – m – ms – s vs vs w vs vs m m – m ms – ms – vw w w vs ms mw m –
3069 3069 3056 3047 sh 1593 1583 1477 1427 1317 1246 1224 1187 1096 1037 1022 816 614 534 398 – 932 904 – 703 498 414 242
Vapor (14) (14) (12) (8) (7) (3) (2) (1) (1) (5) (9) (6) (5) (100) (5) (31) (12) (24) (2) – (1) (1) – (1) (4) (2) (33)
3079 3075 3063 3054 1594 1587 sh 1480 1426 1316 1249 1226 1187 1096 1038 1022 816 613 533 398 – – – – – – – 239
(10) (16) (12) (5) (2) (1) (1) (1) (1) (10) (4) (2) (3) (100) (4) (20) (2) (15) (1) – – – – – – – (6)
Calculatedb
GKPc
Intensity
3088 3073 3054 3047 1602 1596 1482 1438 1323 1265 1223 1197 1107 1041 1020 819 619 533 390 966 932 905 803 702 501 414 231
(2, 100) (9, 70) (7, 65) (14, 54) (8, 31) (20, 28) (60, 10) (52, 7) (1, 2) (3, 10) (131, 41) (3, 21) (12, 10) (0.5, 100) (11, 28) (16, 45) (4, 17) (11, 17) (5, 1) (0.1, 0) (3, 1) (2, 1) (41, 2) (29, 1) (0.2, 1) (3, 1) (0.6, 7)
3069 3058 3069 3058 1594 1584 1480 1425 1308 1247 – 1187 1095 1038 1023 818 616 535 – 982 – – – 702 – 410 244
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; i.p., in-plane; o.p., out-of-plane. a Relative intensities in parenthesis. b B3LYP/6-311++g(d,p); frequencies scaled with a scaling factor of 0.985 for frequencies less than 1800 cm−1 and 0.964 for frequencies greater than 1800 cm−1 . The calculated relative intensities are shown as (IR, Raman). c Ref. [2].
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P. Boopalachandran, J. Laane / Spectrochimica Acta Part A 79 (2011) 1191–1195
x4 N F
Transmittance
calculated
x5
liquid
x4
vapor 3200
1600
cm-1
1200
800
Fig. 4. Liquid, vapor, and calculated infrared spectra of 3-fluoropyridine.
3. Computations
4. Results and discussion
The structures and vibrational frequencies of 2FPy and 3FPy for the electronic ground state were calculated using the Gaussian 07 program package [4]. Ab initio second order Moller–Plesset (MP2) level of theory with the cc-pVTZ basis set was used to find the optimized geometry. The DFT-B3LYP level of theory with the 6-311++G(d,p) basis set was used to calculate the vibrational frequencies and the infrared and Raman intensities. Based on our previous work [5–9], a scaling factor of 0.964 was used for the C–H stretching vibrational frequencies and a factor of 0.985 for the lower frequencies.
4.1. Structures Fig. 1 shows the calculated structures of 2FPy and 3FPy and pyridine in their ground electronic states. As can be seen, the substitution of the fluorine atom on the pyridine ring for the most part has only a minor effect on the ring bond distances and angles. The notable exception is the N–C(F) bond distance for 2FPy which is only 1.313 A˚ as compared to 1.340 A˚ for pyridine and 1.344 A˚ for the other N–C bond of 2FPy. Clearly the substitution of the electronegative fluorine atom results in the strengthening of the adjacent N–C
calculated N
Raman Intensity
F
liquid
x4
vapor x4
3200
2800
1600
1200
cm-1
800
Fig. 5. Liquid, vapor and calculated Raman spectra of 3-fluoropyridine.
400
P. Boopalachandran, J. Laane / Spectrochimica Acta Part A 79 (2011) 1191–1195
1195
Table 3 Vibrational frequencies (cm−1 ) of the ring modes of the fluoro-pyridines compared to pyridine. a
Approximate description
5 6 7 8 10 15 16 17 18 24 25 26
Ring stretch Ring stretch Ring stretch Ring stretch Ring stretch Ring breathing Ring bend (i.p.) Ring bend (i.p.) Ring bend (i.p.) Ring bend (o.p.) Ring bend (o.p.) Ring bend (o.p.)
2FPy 1605 1593 1478 1439 1286 997 842 620 554 733 518 414
3FPy 1594 1588 1480 1426 1249 1022 816 613 533 701 507 412
Pyridine 1584 1576 1483 1443 1227 1031 991 654 601 700 403 375
Abbreviations: i.p., in-plane; o.p., out-of-plane. a Mode number for 2FPy and 3FPy.
bond. There is insignificant effect observed for 3FPy since the fluorine atom is distant from the nitrogen atom. The C–F bond distance is 1.338 A˚ for 2FPy and 1.340 A˚ for 3FPy and these values are very similar to the fluorobenzene value of 1.35 A˚ determined from its microwave spectrum [10].
4.2. Infrared and Raman spectra Figs. 2 and 3 show the liquid-phase, vapor-phase, and calculated infrared and Raman spectra of 2FPy, and Figs. 4 and 5 show the spectra for 3FPy. Table 1 summarizes the data for 2FPy and Supplementary Table S1 presents a tabulation of all the spectral bands including the sum and combination bands. Table 2 summarizes the data for 3FPy and Supplementary Table S2 tabulates all of the observed spectral bands for this molecule. Table 3 compares the vibrational frequencies for the ring modes of 2FPy, 3FPy, and the unsubstituted pyridine. Green and coworkers [2] had previously made partial assignments for the fluoropyridines and these are also shown in Tables 1 and 2. As we have learned to expect [5–9] the cc-PVTZ calculation does a remarkably good job of predicting the frequencies. The average difference between experimental and calculated wavenumbers is less than 7 cm−1 . From Table 3 it is also clear that most of the pyridine ring vibrational frequencies are not changed much in 2FPy and 3FPy and the highest four ring stretching modes shift by less than 15 cm−1 . The B2 1227 cm−1 band of pyridine can be seen to shift to 1286 cm−1 in 2FPy and to 1249 cm−1 in 3FPy. The two A1 stretching modes of pyridine at 1031 and 991 cm−1 shift to 997 and 842 cm−1 in 2FPy and to 1022 and 816 cm−1 in 3FPy. These vibrational shifts for the fluoropyridines reflect interactions with the C–F stretching which occurs at 1266 cm−1 for 2FPy and 1227 cm−1 for 3FPy. The C–F stretching frequencies can be compared to values of 1238 cm−1 for fluorobenzene [11] and 1049 cm−1 for methylfluoride [12]. Thus, it is clear that in fluorobenzene and the fluoropyridines the higher C–F stretching frequencies reflect interactions with the bonding within the rings. It is also noteworthy that the two lowest out-of-plane ring vibrations for 2FPy (at 518 and 414 cm−1 ) are somewhat higher than those for pyridine (403 and 375 cm−1 ). This is an indication that the fluoropyridines are also rigid in their electronic ground state and somewhat more so than pyridine itself. As we reported previously [1], in its S1 (n,*) excited state pyridine becomes very floppy. There is also little difference between the pyridine and fluoropyridine vibrational frequencies for the C–H stretches (3030–3095 cm−1 ), the in-plane C–H wags (1070–1365 cm−1 ), and the out-of-plane C–H wags (730–1000 cm−1 ).
5. Conclusions The structures of 2FPy and 3FPy have been calculated and the ring bond distances differ little from those of pyridine. The notable exception is that the N–C(F) bond distance is shortened in 2FPy due to interactions. The frequencies of the ring modes of the fluoropyridines are also similar to those of pyridine itself. The C–F stretching frequencies at 1266 cm−1 for 2FPy and 1227 cm−1 for 3FPy reflect bond strengths similar to that in fluorobenzene where (C–F) is 1238 cm−1 . Acknowledgements The authors wish to thank the Robert A. Welch Foundation (Grant A-0396) for financial support. Calculations were carried out on the Texas A&M Department of Chemistry Medusa computer system funded by the National Science Foundation, Grant No. CHE0541587. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2011.04.041. References [1] P. Boopalachandran, J. Laane, Chem. Phys. Lett. 462 (2008) 178. [2] H.S. Green, W. Kynaston, H.M. Paisley, Spectrochim. Acta 19 (1963) 549. [3] K. Haller, W.Y. Chiang, A. Rosario, J. Laane, J. Mol. Struct. 379 (1996) 19. [4] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, revision C.02, Gaussian, Inc., Wallingford, CT, 2004. [5] J. Yang, K. McCann, J. Laane, J. Mol. Struct. 695–696 (2004) 339. [6] J. Yang, J. Choo, O. Kwon, J. Laane, Spectrochim. Acta Part A 68 (2007) 1170. [7] D. Autrey, J. Yang, J. Laane, J. Mol. Struct. 661 (2003) 23. [8] A.A. Al-Saadi, J. Laane, J. Mol. Struct. 830 (2007) 46. [9] D. Autrey, J. Choo, J. Laane, J. Phys. Chem. A 105 (2001) 10230. [10] L. Nygaard, I. Bojesen, T. Pedersen, J. Rastrup-Anderson, J. Mol. Struct. 2 (1968) 209. [11] E.D. Lipp, J. Seliskar, J. Mol. Spectrosc. 73 (1978) 290. [12] S. Kondo, Y. Koga, T. Nakanaga, J. Phys. Chem. 90 (1986) 1519.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Vibrational spectra, structure, and theoretical calculations of 2-fluoro- and 3-fluoropyridine Praveenkumar Boopalachandran, Jaan Laane ∗ Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, United States
a r t i c l e
i n f o
Article history: Received 3 March 2011 Received in revised form 23 March 2011 Accepted 15 April 2011 Keywords: Infrared Raman Ab initio DFT Fluoropyridine Structure
a b s t r a c t The infrared and Raman spectra of liquid and vapor-phase 2-fluoropyridine and 3-fluoropyridine have been recorded and assigned. Ab initio and DFT calculations were carried out to compute the molecular structures and to verify the vibrational assignments. The observed and calculated spectra agree extremely well. The ring bond distances of the fluoropyridines are very similar to those of pyridine except for a shortening of the C–N(F) bond in 2-fluoropyridine. The C–F bond stretching frequencies are similar to that in fluorobenzene reflecting the influence of the ring bonding. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In 2008 we reported the ultraviolet absorption spectrum of pyridine and showed that the molecule is very floppy and quasiplanar in its S1 (n,*) electronic excited state [1]. Determination of the potential energy function for the ring-bending vibration showed the barrier to planarity to be only 3 cm−1 . This is in contrast to the S0 ground state where pyridine is rigidly planar. In a continuation of our work on the vibrations and structure of pyridine and substituted pyridines in their ground and excited states, in the work described here we present our results on two substituted fluoropyridines in their electronic ground state. The infrared and Raman spectra of 2-fluoropyridine (2FPy) and 3fluoropyridine (3FPy) have been analysed and their vibrational frequencies have been determined. Ab initio and DFT computations were carried out to compute the structures of these molecules and to complement the experimental work and confirm the vibrational assignments. Green and co-workers [2] have previously reported the infrared and Raman spectra and presented partial assignments for these molecules for the electronic ground state, but no structural information has been previously reported.
2-Fluoropyridine (2FPy) and 3-fluoropyridine (3FPy) (99% purity) were purchased from Aldrich and purified by trap to trap distillation. The Raman spectra of the molecules in the vapor-phase were recorded of samples sealed in specially designed glass cells which have previously been described [3]. The vapor pressures of the samples at room temperature were about 9 Torr for 2FPy and 15 Torr for 3FPy. A Jobin-Yvon U-1000 spectrometer equipped with a liquid nitrogen-cooled CCD detector was used to collect the spectra. The 532 nm line of a frequency-doubled Nd:YAG Coherent Verdi-10 laser was used and typically operated at 5 W of power. Spectral scans spanning 60 cm−1 were typically recorded over periods of 4–6 h so that many hundreds of individual spectra could be averaged. The spectral resolution was 0.7 cm−1 . The liquid phase Raman spectra were also collected on the same instrument with samples in glass cuvettes using a laser power of 500 mW. The liquid-phase and vapor-phase mid-infrared spectra of 2FPy and 3FPy were collected on a Bruker Vertex 70 FT spectrometer equipped with a globar light source, a KBr beamsplitter and deuterated lanthanum triglycine sulfate (DLaTGS) detector. The vapor-phase far infrared spectra (60–600 cm−1 ) were also collected on the same instrument equipped with a mylar beamsplitter, and a mercury cadmium telluride (MCT) detector. The vapor pressures of the samples were the same as for the Raman measurements. Typically 1024 scans were collected using a resolution of 0.5 cm−1 .
∗ Corresponding author. E-mail address: [email protected] (J. Laane). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.04.041
1192
P. Boopalachandran, J. Laane / Spectrochimica Acta Part A 79 (2011) 1191–1195
Fig. 1. Calculated structures of (a) pyridine, (b) 2-fluoropyridine, and (c) 3-fluoropyridine.
x4
N
F
Transmittance
calculated
x4
liquid
x4
vapor 3000
1600
cm-1
1200
800
Fig. 2. Liquid, vapor, and calculated infrared spectra of 2-fluoropyridine.
calculated
Raman Intensity
N
liquid
x4
vapor
3200
2800
1600
1200
cm-1
800
Fig. 3. Liquid, vapor and calculated Raman spectra of 2-fluoropyridine.
400
F
P. Boopalachandran, J. Laane / Spectrochimica Acta Part A 79 (2011) 1191–1195
1193
Table 1 Observed and calculated vibrational frequencies (cm−1 ) for 2-fluoropyridine. Cs
Approximate description
A (i.p.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
C–H stretch C–H stretch C–H stretch C–H stretch Ring stretch Ring stretch Ring stretch Ring stretch C–H wag Ring stretch C–F stretch C–H wag C–H wag C–H wag Ring breathing Ring bend Ring bend Ring bend C–F wag C–H wag C–H wag C–H wag C–H wag Ring twist Ring bend Ring bend C–F wag
Ramana
Infrared Liquid
A (o.p.)
3100 sh 3100 sh 3086 3069 1598 1581 1473 1436 1301 1295 1258 br 1143 1098 1045 994 840 621 554 433 983 963 873 780 733 519 417 –
Vapor w w mw mw vs vs vs vs m m vs s m m s vs ms s w w w w vs m m m –
3100.1 3092.4 3080.9 – 1604.5 1593.0 1477.6 1438.7 1302.9 1286.3 1265.9 1139.4 1097.9 1044 sh 996.6 842.3 620.1 553.8 432.3 – 960.5 868.2 780.4 732.5 517.7 413.8 –
Liquid s s s – s vs vvs vvs m m vs vs mw m mw vs w m vw – mw mw ms m m mw –
3101 br 3101 br 3084 3071 1596 1579 1471 1434 1301 – 1259 br 1141 1096 1044 993 839 620 553 431 982 962 870 783 733 517 416 228
Vapor (6) (6) (14) (26) (5) (4) (1) (0.5) (6) – (6) (2) (8) (41) (100) (25) (11) (31) (3) (3) (0.1) (0.5) (0.6) (1) (2) (0.6) (29)
3100 br 3092 3080 3077 – – 1478 1439 1303 1286 1265 1139 1098 1045 999 842 620 554 433 982 963 869 – – – – 226
(9) (42) (56) (60) – – (2) (2) (18) (7) (22) (2) (7) (182) (100) (125) (6) (40) (7) (2) (1) (1) – – – – (20)
Calculatedb
GKPc
Intensity
3096 3086 3066 3050 1610 1597 1480 1441 1306 1289 1247 1148 1100 1046 995 834 625 551 427 979 962 871 781 732 517 419 216
(0.7, 100) (10, 70) (6, 44) (10, 55) (61, 43) (79, 43) (78, 5) (79, 2) (3, 19) (2, 14) (140, 62) (6, 10) (3, 24) (7, 71) (6, 100) (36, 62) (2, 19) (6, 24) (0.6, 0.1) (0.1, 0.4) (1, 0.1) (1, 0.1) (70, 1) (6, 1) (4, 2) (4, 0.4) (0, 10)
3097 3094 3074 3074 1598 1580 – – 1303 – 1249 1146 1099 1045 996 828 622 556 – – – – – – – – 230
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; i.p., in-plane; o.p., out-of-plane. a Relative intensities in parenthesis. b B3LYP/6-311++g(d,p); frequencies scaled with a scaling factor of 0.985 for frequencies less than 1800 cm−1 and 0.964 for frequencies greater than 1800 cm−1 . The calculated relative intensities are shown as (IR, Raman). c Ref. [2].
Table 2 Observed and calculated vibrational frequencies (cm−1 ) for 3-fluoropyridine. Cs
Approximate description
Infrared
A (i.p.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
C–H stretch C–H stretch C–H stretch C–H stretch Ring stretch Ring stretch Ring stretch Ring stretch C–H wag Ring stretch C–F stretch C–H wag C–H wag C–H wag Ring breathing Ring bend Ring bend Ring bend C–F wag C–H wag C–H wag C–H wag C–H wag Ring twist Ring bend Ring bend C–F wag
– 3070 – 3049 1594 1583 1478 1427 1319 1248 1227 1189 1098 1038 sh 1024 816 sh 615 535 – 981 938 905 804 702 497 415 –
Ramana
Liquid
A (o.p.)
Vapor – m – m m s vs vs w vs vs m ms w ms ms mw s – vw w w vs s w mw –
– 3076.5 – 3054.1 – 1588.4 1480.1 1425.8 1315.6 1249.4 1227.4 1187.1 1096.0 – 1021.8 816.4 – 533.3 – 974.1 934.0 905.1 800.7 701.0 506.8 411.7 –
Liquid – m – ms – s vs vs w vs vs m m – m ms – ms – vw w w vs ms mw m –
3069 3069 3056 3047 sh 1593 1583 1477 1427 1317 1246 1224 1187 1096 1037 1022 816 614 534 398 – 932 904 – 703 498 414 242
Vapor (14) (14) (12) (8) (7) (3) (2) (1) (1) (5) (9) (6) (5) (100) (5) (31) (12) (24) (2) – (1) (1) – (1) (4) (2) (33)
3079 3075 3063 3054 1594 1587 sh 1480 1426 1316 1249 1226 1187 1096 1038 1022 816 613 533 398 – – – – – – – 239
(10) (16) (12) (5) (2) (1) (1) (1) (1) (10) (4) (2) (3) (100) (4) (20) (2) (15) (1) – – – – – – – (6)
Calculatedb
GKPc
Intensity
3088 3073 3054 3047 1602 1596 1482 1438 1323 1265 1223 1197 1107 1041 1020 819 619 533 390 966 932 905 803 702 501 414 231
(2, 100) (9, 70) (7, 65) (14, 54) (8, 31) (20, 28) (60, 10) (52, 7) (1, 2) (3, 10) (131, 41) (3, 21) (12, 10) (0.5, 100) (11, 28) (16, 45) (4, 17) (11, 17) (5, 1) (0.1, 0) (3, 1) (2, 1) (41, 2) (29, 1) (0.2, 1) (3, 1) (0.6, 7)
3069 3058 3069 3058 1594 1584 1480 1425 1308 1247 – 1187 1095 1038 1023 818 616 535 – 982 – – – 702 – 410 244
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; i.p., in-plane; o.p., out-of-plane. a Relative intensities in parenthesis. b B3LYP/6-311++g(d,p); frequencies scaled with a scaling factor of 0.985 for frequencies less than 1800 cm−1 and 0.964 for frequencies greater than 1800 cm−1 . The calculated relative intensities are shown as (IR, Raman). c Ref. [2].
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x4 N F
Transmittance
calculated
x5
liquid
x4
vapor 3200
1600
cm-1
1200
800
Fig. 4. Liquid, vapor, and calculated infrared spectra of 3-fluoropyridine.
3. Computations
4. Results and discussion
The structures and vibrational frequencies of 2FPy and 3FPy for the electronic ground state were calculated using the Gaussian 07 program package [4]. Ab initio second order Moller–Plesset (MP2) level of theory with the cc-pVTZ basis set was used to find the optimized geometry. The DFT-B3LYP level of theory with the 6-311++G(d,p) basis set was used to calculate the vibrational frequencies and the infrared and Raman intensities. Based on our previous work [5–9], a scaling factor of 0.964 was used for the C–H stretching vibrational frequencies and a factor of 0.985 for the lower frequencies.
4.1. Structures Fig. 1 shows the calculated structures of 2FPy and 3FPy and pyridine in their ground electronic states. As can be seen, the substitution of the fluorine atom on the pyridine ring for the most part has only a minor effect on the ring bond distances and angles. The notable exception is the N–C(F) bond distance for 2FPy which is only 1.313 A˚ as compared to 1.340 A˚ for pyridine and 1.344 A˚ for the other N–C bond of 2FPy. Clearly the substitution of the electronegative fluorine atom results in the strengthening of the adjacent N–C
calculated N
Raman Intensity
F
liquid
x4
vapor x4
3200
2800
1600
1200
cm-1
800
Fig. 5. Liquid, vapor and calculated Raman spectra of 3-fluoropyridine.
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1195
Table 3 Vibrational frequencies (cm−1 ) of the ring modes of the fluoro-pyridines compared to pyridine. a
Approximate description
5 6 7 8 10 15 16 17 18 24 25 26
Ring stretch Ring stretch Ring stretch Ring stretch Ring stretch Ring breathing Ring bend (i.p.) Ring bend (i.p.) Ring bend (i.p.) Ring bend (o.p.) Ring bend (o.p.) Ring bend (o.p.)
2FPy 1605 1593 1478 1439 1286 997 842 620 554 733 518 414
3FPy 1594 1588 1480 1426 1249 1022 816 613 533 701 507 412
Pyridine 1584 1576 1483 1443 1227 1031 991 654 601 700 403 375
Abbreviations: i.p., in-plane; o.p., out-of-plane. a Mode number for 2FPy and 3FPy.
bond. There is insignificant effect observed for 3FPy since the fluorine atom is distant from the nitrogen atom. The C–F bond distance is 1.338 A˚ for 2FPy and 1.340 A˚ for 3FPy and these values are very similar to the fluorobenzene value of 1.35 A˚ determined from its microwave spectrum [10].
4.2. Infrared and Raman spectra Figs. 2 and 3 show the liquid-phase, vapor-phase, and calculated infrared and Raman spectra of 2FPy, and Figs. 4 and 5 show the spectra for 3FPy. Table 1 summarizes the data for 2FPy and Supplementary Table S1 presents a tabulation of all the spectral bands including the sum and combination bands. Table 2 summarizes the data for 3FPy and Supplementary Table S2 tabulates all of the observed spectral bands for this molecule. Table 3 compares the vibrational frequencies for the ring modes of 2FPy, 3FPy, and the unsubstituted pyridine. Green and coworkers [2] had previously made partial assignments for the fluoropyridines and these are also shown in Tables 1 and 2. As we have learned to expect [5–9] the cc-PVTZ calculation does a remarkably good job of predicting the frequencies. The average difference between experimental and calculated wavenumbers is less than 7 cm−1 . From Table 3 it is also clear that most of the pyridine ring vibrational frequencies are not changed much in 2FPy and 3FPy and the highest four ring stretching modes shift by less than 15 cm−1 . The B2 1227 cm−1 band of pyridine can be seen to shift to 1286 cm−1 in 2FPy and to 1249 cm−1 in 3FPy. The two A1 stretching modes of pyridine at 1031 and 991 cm−1 shift to 997 and 842 cm−1 in 2FPy and to 1022 and 816 cm−1 in 3FPy. These vibrational shifts for the fluoropyridines reflect interactions with the C–F stretching which occurs at 1266 cm−1 for 2FPy and 1227 cm−1 for 3FPy. The C–F stretching frequencies can be compared to values of 1238 cm−1 for fluorobenzene [11] and 1049 cm−1 for methylfluoride [12]. Thus, it is clear that in fluorobenzene and the fluoropyridines the higher C–F stretching frequencies reflect interactions with the bonding within the rings. It is also noteworthy that the two lowest out-of-plane ring vibrations for 2FPy (at 518 and 414 cm−1 ) are somewhat higher than those for pyridine (403 and 375 cm−1 ). This is an indication that the fluoropyridines are also rigid in their electronic ground state and somewhat more so than pyridine itself. As we reported previously [1], in its S1 (n,*) excited state pyridine becomes very floppy. There is also little difference between the pyridine and fluoropyridine vibrational frequencies for the C–H stretches (3030–3095 cm−1 ), the in-plane C–H wags (1070–1365 cm−1 ), and the out-of-plane C–H wags (730–1000 cm−1 ).
5. Conclusions The structures of 2FPy and 3FPy have been calculated and the ring bond distances differ little from those of pyridine. The notable exception is that the N–C(F) bond distance is shortened in 2FPy due to interactions. The frequencies of the ring modes of the fluoropyridines are also similar to those of pyridine itself. The C–F stretching frequencies at 1266 cm−1 for 2FPy and 1227 cm−1 for 3FPy reflect bond strengths similar to that in fluorobenzene where (C–F) is 1238 cm−1 . Acknowledgements The authors wish to thank the Robert A. Welch Foundation (Grant A-0396) for financial support. Calculations were carried out on the Texas A&M Department of Chemistry Medusa computer system funded by the National Science Foundation, Grant No. CHE0541587. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2011.04.041. References [1] P. Boopalachandran, J. Laane, Chem. Phys. Lett. 462 (2008) 178. [2] H.S. Green, W. Kynaston, H.M. Paisley, Spectrochim. Acta 19 (1963) 549. [3] K. Haller, W.Y. Chiang, A. Rosario, J. Laane, J. Mol. Struct. 379 (1996) 19. [4] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, revision C.02, Gaussian, Inc., Wallingford, CT, 2004. [5] J. Yang, K. McCann, J. Laane, J. Mol. Struct. 695–696 (2004) 339. [6] J. Yang, J. Choo, O. Kwon, J. Laane, Spectrochim. Acta Part A 68 (2007) 1170. [7] D. Autrey, J. Yang, J. Laane, J. Mol. Struct. 661 (2003) 23. [8] A.A. Al-Saadi, J. Laane, J. Mol. Struct. 830 (2007) 46. [9] D. Autrey, J. Choo, J. Laane, J. Phys. Chem. A 105 (2001) 10230. [10] L. Nygaard, I. Bojesen, T. Pedersen, J. Rastrup-Anderson, J. Mol. Struct. 2 (1968) 209. [11] E.D. Lipp, J. Seliskar, J. Mol. Spectrosc. 73 (1978) 290. [12] S. Kondo, Y. Koga, T. Nakanaga, J. Phys. Chem. 90 (1986) 1519.