Microwave Rotational Spectra and Structures of 2-Fluoropyridine and

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Microwave Rotational Spectra and Structures of 2-Fluoropyridine and 3-Fluoropyridine Cody W. van Dijk, Ming Sun, and Jennifer van Wijngaarden* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada S Supporting Information *

ABSTRACT: The ground state rotational spectra of 2-fluoropyridine and 3-fluoropyridine have been investigated using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopies. In addition to the parent species, the spectra of the 13C and 15N singly substituted isotopologues were recorded in the 8−23 GHz region in natural abundance. The rotational constants determined for the seven isotopologues of each were used to calculate relevant geometric parameters including the bond distances and angles of the pyridine ring backbone. The derived structures show a more pronounced deviation from the pyridine ring geometry when the fluorine substituent is ortho to nitrogen which is consistent with ab initio predictions at various levels of theory. Analysis of the 14N hyperfine structure provided an additional source of information about the electronic structure surrounding the nitrogen atom as a function of fluorine substitution. Together, the experimental results are consistent with a bonding model that involves hyperconjugation whereby fluorine donates electron density from its lone pair into the π-system of pyridine.

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0.05 MHz accuracy, and the 14N hyperfine splittings were partially resolved. As only the main isotopologue of each was investigated, the structures were estimated by varying only the C−F bond length and FCN angle to match the experimental rotational constants while all other parameters were fixed to the values for pyridine. Since these first experiments, the structures of 2-fluoropyridine and 3-fluoropyridine have been investigated using various ab initio methods.5−9 It was found that the valence angle of the ring at the site of fluorination increases by a few degrees (while neighboring angles decrease). Furthermore, the calculations suggest fluorination at the ortho position (2fluoropyridine) relative to nitrogen has a larger effect on the geometry of the pyridine ring backbone than substitution at the meta position (3-fluoropyridine) suggesting that the former perturbs the π-system of the ring to a greater extent. In this paper, we report the high resolution pulsed-jet Fourier transform microwave (FTMW) spectra of 2-fluoropyridine and 3-fluoropyridine along with those of six heavy atom isotopologues in natural abundance (15N 0.36%, 13C 1.07%). The data set includes heavy atom substitution at each unique site within the rings. Transitions arising from the minor isotopologues are reported here for the first time while the data sets for the parent species have been extended to higher frequencies and measured more precisely than in the earlier microwave studies.3,4 This enabled the accurate experimental determination of the structural parameters of the ring backbone

INTRODUCTION The substitution of a hydrogen atom with fluorine in an organic compound can have significant effects on the molecule’s physical and chemical properties because although the atoms have similar van der Waals radii, their electronic properties are quite different. Fluorination is used to alter the rate of a reaction, and this phenomenon has been exploited to tune the pharmacokinetic properties of drugs. Due to its small radius, there is also interest in using fluorine directly as a tag to follow the metabolic pathways in vivo and in vitro for toxicology and pharmaceutical investigations.1 Such studies require fluorine tagging to be carried out with little structural change to the molecule. High resolution microwave spectroscopy is a valuable tool for probing the geometry and electronic structure of a molecule and can thus be used to monitor the effect of fluorination on molecular properties. Pyridine (C5H5N) is an aromatic heterocyclic compound, and a large number of its derivatives have found applications in chemistry. Its radioactive fluorine-18 analogues, for example, have found use in positron emission tomography (PET) to follow the dynamics of drugs in vivo.2 Fluorinated pyridines provide interesting prototypical systems to study the effect of fluorine substitution via microwave spectroscopy. They have appreciable dipole moments, they are amenable to high level ab initio calculations, and the 14N nucleus provides a local probe of the electronic environment in the ring. In the case of the monofluorinated species, 2-fluoropyridine and 3-fluorpyridine, the ground state rotational spectra of the parent species were reported in the 1970s using a Stark modulated microwave spectrometer.3,4 Transitions were measured to approximately © 2012 American Chemical Society

Received: February 23, 2012 Revised: March 22, 2012 Published: March 24, 2012 4082

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Figure 1. Structure of (a) 2-fluoropyridine and (b) 3-fluoropyridine in their principal axis systems.

A sample cp-FTMW spectrum is shown in Figure 2 in the region of the 202−101 transition of the parent moiety and the six

to allow comparison of structural changes with respect to fluorination at different sites. More complete analyses of the 14 N hyperfine splittings provided insight into deviations in the electronic structure that accompany fluorine substitution at the ortho and meta positions. The results of our spectroscopic investigation may be explained in terms of a hyperconjugation model involving the donation of electron density from fluorine into the ring system.

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EXPERIMENTAL SECTION The pure rotational spectra of the parent and 13C and 15N isotopologues were first recorded using our broadband chirped pulse Fourier transform microwave (cp-FTMW) instrument which has been previously described.10 Survey spectra were typically measured in 2 GHz windows between 8 and 14 GHz, and the observed transitions had linewidths of ∼200 kHz fwhm (full width at half-maximum) when individual FIDs are acquired for 20 μs. Samples of each compound were prepared as a gas mixture of 0.04−0.08% fluoropyridine in argon and delivered to the pulsed nozzle with a backing pressure of 50 psi. These initial survey spectra guided our experiments using the more sensitive Balle−Flygare FTMW spectrometer11 which was used to record individual hyperfine components with higher resolution (∼7 kHz fwhm) and to seek weaker transitions. Gas mixtures for the FTMW measurements were more dilute (0.03%) and maintained at a higher backing pressure (80 psi) to optimize the signal. The spectra of all minor isotopologues were observed in natural abundance (13C 1.07%, 15N 0.36%).

Figure 2. Sample cp-FTMW spectrum of the 202−101 transitions of seven different isotopologues of 2-fluoropyridine. The 422−413 transition of the parent molecule is also seen in this region. The spectrum was recorded with 20 000 gas pulses with 15 FIDs recorded for each.

minor isotopologues of 2-fluoropyridine. The 14N nuclear quadrupole hyperfine structure of most transitions was not resolved with this instrument. Consequently, additional measurements were made using our higher resolution Balle−Flygare FTMW spectrometer, and an example of the observed hyperfine splitting for the 414−313 rotational transition is shown in Figure 3. In total, 33 a-type and b-type rotational transitions of the parent moiety were measured between 8 and 23 GHz yielding 106 resolved hyperfine components due to the quadrupole 14N nucleus. The observed transitions in this region span quantum numbers J′ = 2 through J′ = 7. Initial attempts to fit the hyperfine structure using the quadrupole coupling constants χaa and χbb−χcc led to larger than expected residuals for the higher Ka transitions (5−20 kHz). The 14N hyperfine constants of 2-fluoropyridine were subsequently calculated in Gaussian03 following geometry optimization (Møller−Plesset MP2, 6311G++2d2p) using the keyword Pickett.12 This revealed a rather large predicted value of χab (2.77 MHz). After including this additional parameter in Watson’s A-reduced Hamiltonian (Ir representation) in Pickett’s spectral fitting program, all observed hyperfine components were well determined in the fit with most having values of obs − calc within ±1 kHz.

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SPECTRAL ASSIGNMENT AND ANALYSIS Using the rotational constants of 2-fluoropyridine and 3fluoropyridine reported by Sharma et al.3,4 as a guide, we measured the spectra of the normal species using our two pulsed-jet FTMW spectrometers to extend the data sets and obtain a more complete assignment of the 14N hyperfine splittings. Our reinvestigation of the parent molecules formed the basis for our subsequent investigation of the 13C and 15N isotopologues which were not previously reported. The isotopologues studied are those involving heavy atom substitution at each of the five unique carbon sites in the ring as well as at the nitrogen center. 2-Fluoropyrindine. On the basis of the Stark measurements of Sharma et al.,3 2-fluoropyridine has significant dipole components along the a- and b-principal inertial axes (μa = 2.8 D and μb = 1.9 D) which give rise to strong a-type and b-type transitions in the microwave region. The molecule is shown in its principal inertial axis system in Figure 1. 4083

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0.149(3) 0.25(2) 0.922 0.0488 0.371 1.8 55 0.149(3) 0.20(2) 0.922 0.0488 0.371 1.9 53 0.153(3) 0.25(4) 0.922 0.0488 0.371 1.6 53 0.146(3) 0.220(12) 0.89(3) 0.0488 0.371 1.2 30

−0.140(3) −1.3977(15) −0.158(3) −1.3981(16)

−0.268(3) −1.3752(16)

5769.5965(6) 2699.736 99(18) 1838.955 75(15)

C3

13

C2

13

normal

15

N

5779.4641(3) 2699.096 38(11) 1839.658 57(11)

a

DISCUSSION The rotational constants of 2-fluoropyridine and 3-fluoropyridine are in good agreement with the previously reported values for the parent species3,4 and with those calculated at the MP2 level of theory (6-311-G++2d2p) as shown in Tables 1 and 2. The inertial defects (Δo) are small: 0.029 amu Å2 and 0.041 amu Å2, respectively, as expected for planar molecules.

Rotational Constants /MHz A 5870.881 05(19) B 2699.985 71(10) C 1849.242 59(7) 14 N Nuclear Quadrupole Coupling Constantsc/MHz −0.1504(15) 1.5(χaa) −1.3932(4) 0.25(χbb−χcc) 2.76(5) χab Centrifugal Distortion Constantsd /kHz ΔJ 0.149(2) ΔJK 0.232(8) ΔK 0.922(19) 0.0488(11) δj δk 0.371(11) rms/kHz 1.2 no. lines 106

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a,b

Table 1. Ground State Spectroscopic Constants of 2-Fluoropyridine (A Reduction, Ir Representation)

13

The resulting spectroscopic constants for 2-fluoropyridine are provided in Table 1 along with those obtained from fitting the spectra of the 13C and 15N isotopologues. As fewer transitions were observed for the minor species, some centrifugal distortion parameters were held fixed to the values obtained from fitting the parent spectrum of 2-fluoropyridine as denoted in Table 1. Only lower Ka transitions were observed for the 13 C species due to the low intensity of the higher Ka transitions. As a result, the hyperfine patterns were not sensitive to χab, and thus, the χab parameter was not included in the fit for these isotopologues. For each of the seven isotopologues of 2-fluoropyridine studied, the root-mean-square (rms) error of the fit was less than 2 kHz. The observed transitions frequencies are provided as Supporting Information. 3-Fluoropyridine. For 3-fluoropyridine, the two in-plane dipole components are appreciable (μa = 0.8 D and μb = 2.2 D), and the a-type transitions are predicted to be weaker than the b-type transitions.4 Compared with its structural isomer in Figure 1, the center of mass and orientation of the principal inertial axes are quite different due to the position of the heavy fluorine atom. The assignment and analysis of the spectra of the isotopologues of 3-fluoropyridine followed that described for 2-fluoropyridine. A total of 112 hyperfine components due to 39 a-type and b-type rotational transitions were measured between 9 and 23 GHz for the parent molecule. The observed transitions in this region span quantum numbers J′ = 2 through J′ = 8. As in the previous case, ab initio calculations (MP2, 6-311G++2d2p) predicted a substantial value of χab (−2.74 MHz) for 3-fluoropyridine, and this parameter was included in the fit. In this case, the 13C spectra were slightly sensitive to the inclusion of this parameter; however, the degree of uncertainty in the determined χab values were isotopologue-dependent. The spectroscopic constants obtained using Watson’s A-reduced Hamiltonian (Ir representation) are provided in Table 2 for 3-fluoropyridine and its minor isotopologues. For each fit, the rms error was less than 2 kHz, and the observed transitions frequencies are provided as Supporting Information.

5871.2147(7) 2690.643 98(18) 1844.890 37(16)

C4

5778.6121(5) 2677.959 03(17) 1829.725 61(16)

Figure 3. Sample Balle−Flygare FTMW spectrum of the 414−313 transition of 2-fluoropyridine showing the hyperfine splitting arising due to the 14N quadrupole nucleus.

Rotational constants from ref 3: A = 5870.883(14) MHz, B = 2699.977(6) MHz, C = 1849.246(4) MHz. bCalculated rotational constants (MP2/6-311G++2d2p) from this work: A = 5880.5 MHz, B = 2688.3 MHz, C = 1844.9 MHz. cCalculated 14N hyperfine constants (MP2/6-311G++2d2p) from this work: 1.5(χaa) = −0.16 MHz, 0.25(χbb-χcc) = −1.42 MHz, χab = 2.77 MHz. dSome centrifugal distortion constants for minor isotopolgues were held fixed to the parent values during the fit. These are given here without uncertainties.

0.155(3) 0.206(19) 0.922 0.0488 0.371 1.8 57 0.144(4) 0.24(3) 0.922 0.0488 0.371 2.2 54

−0.044(3) −1.4104(15) −0.155(4) −1.3979(19)

13

C5

5870.8586(6) 2650.5411(2) 1825.9129(2)

13

C6

5775.7229(5) 2682.964 09(16) 1831.771 13(16)

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Rotational constants from ref 4: A = 5829.661(20) MHz, B = 2637.465(7) MHz, C = 1815.648(8) MHz. bCalculated rotational constants (MP2/6-311G++2d2p) from this work: A = 5835.5 MHz, B = 2629.6 MHz, C = 1812.7 MHz. cCalculated 14N hyperfine constants (MP2/6-311G++2d2p) from this work: 1.5(χaa)=0.02 MHz, 0.25(χbb−χcc) = −1.72 MHz, χab = −2.74 MHz. dSome centrifugal distortion constants for minor isotopolgues were held fixed to the parent values during the fit. These are given here without uncertainties.

The spectra for the six minor isotopologues (15N and 13C) of each moiety provide additional information regarding heavy atom substitution at each site in the pyridine ring. The complete set of rotational constants was subsequently used to derive structural parameters for both 2-fluoropyridine and 3-fluoropyridine as described below. This is followed by a discussion of the 14N nuclear quadrupole structure as a function of fluorine substitution at the ortho and meta positions of the pyridine ring. rs Substitution Geometry. Assuming planarity, a Kraitchman analysis13 was performed to obtain the coordinates and corresponding Costain errors14 of the six heavy atoms in 2fluoropyridine and 3-fluoropyridine using the KRA program.15 From the Kraitchman coordinates, the six unique bond lengths and six valence angles describing the pyridine ring backbone were determined geometrically. Note that while the Kraitchman equations provide only absolute values of the coordinates, the signs are inferred on the basis of the orientation of each molecule in its principal axis system based on the ab initio predictions. The procedure was repeated for pyridine itself using the rotational constants provided in ref 16. The resulting rs geometric parameters of pyridine, 2-fluoropyridine, and 3fluoropyridine are summarized in Table 3. ro Geometry. The ro geometries of 2-fluoropyrdine and 3fluoropyridine were obtained by fitting the 14 experimentally determined A and B rotational constants of each (from the seven isotopologues) to nine key structural parameters involving the heavy atoms of the ring using the STRFIT program.15 In this procedure, the geometric parameters involving the hydrogen and fluorine atoms were fixed at the values obtained via ab initio geometry optimization (MP2 6-311G++2d2p). The structural parameters fit are those listed in boldface in Table 3 under the headings ro. The maximum discrepancy between the observed and calculated rotational constants from this least-squares fitting procedure was only 0.004%. The other bond lengths and angles shown in Table 3 were calculated from the fit parameters using trigonometric relationships. The process was repeated for pyridine using the previously reported microwave spectra,16 and these values are included in Table 3 for comparison. The ro structural parameters for the ring backbone of pyridine, 2-fluoropyridine, and 3-fluoropyridine are compared in Table 3 to the theoretical equilibrium values (re) derived using MP2 theory (6-311G++2d2p). All bond lengths and angles match the calculated values to within the experimental uncertainties from the least-squares fitting method. As first predicted by Boggs and Pang,6 the experimental results derived via microwave spectroscopy show that the valence angle centered at the fluorinated carbon atom increases by roughly 3° when compared to that of pyridine. For example, the NC2C3 angle in pyridine 123.7(5)o becomes 126.4(1)o in 2-fluoropyridine, and the C2C3C4 angle in pyridine 118.5(6)o increases to 121.4(2)o in 3-fluoropyridine, while adjacent valence angles in the ring decrease slightly to account for this change. This may be explained using a distorted hybridization model known as Bent’s rule which states that “atomic s character tends to concentrate in orbitals that are directed toward electropositive groups and atomic p character tends to concentrate in orbitals that are directed toward electronegative groups”.17 This model has been used to describe deviations from tetrahedral symmetry in molecules of type MX2Y2.18 In applying Bent’s rule to the case of the fluoropyridines, the electronegative fluorine atom perturbs the hybridization at the adjacent carbon. A higher electron

a

0.141(3) 0.217(18) 0.97(2) 0.0450 0.416 1.6 60 0.143(4) 0.31(2) 0.85(3) 0.0450 0.416 1.9 60 0.144(3) 0.30(2) 0.85(3) 0.0450 0.416 1.6 60 0.141(3) 0.296(17) 0.89(2) 0.0450 0.416 1.6 58 0.143(3) 0.297(17) 0.87(2) 0.0450 0.416 1.4 60

Rotational Constants /MHz A 5829.701 95(13) B 2637.491 03(7) C 1815.656 19(6) 14 N Nuclear Quadrupole Coupling Constantsc/MHz −0.0520(15) 1.5(χaa) 0.25(χbb−χcc) −1.7416(4) χab −2.751(7) Centrifugal Distortion Constantsd /kHz ΔJ 0.1411(17) ΔJK 0.296(6) ΔK 0.869(17) δj 0.0450(5) δk 0.416(7) rms/kHz 1.3 no. lines 112

0.136(5) 0.30(3) 0.87(4) 0.0450 0.416 1.6 23

−0.060(2) −1.7422(7) −1.5(8) 0.060(3) −1.7603(8) −3.3(3) −0.055(3) −1.7414(7) −2.58(9) −0.0427(18) −1.7436(10) −2.74(4)

−0.075(2) −1.7390(7) −2.5(2)

5829.9770(3) 2627.645 95(15) 1811.013 79(12) 5737.4876(4) 2617.9930(2) 1797.436 86(17)

5738.8248(2) 2637.316 34(11) 1806.660 88(9)

C3 13

C2

13

N normal

15 a,b

Table 2. Ground State Spectroscopic Constants of 3-Fluoropyridine (A Reduction, Ir Representation)

13

C4

5729.2671(2) 2636.958 80(13) 1805.543 53(11)

13

C5

5734.5890(3) 2618.254 06(16) 1797.275 50(13)

13

C6

5829.9437(3) 2592.192 15(13) 1794.097 82(11)

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Table 3. Substitution (rs)a and Effective (ro)b Structural Parameters of Pyridine, 2-Fluoropyridine, and 3-Fluoropyridine along with ab Initio (re) Values (MP2/6-311G++2d2p) Determined in this Work with Bond Lengths in Ångstroms and Angles in Degrees pyridinec N−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−N ∠(N−C2-C3) ∠(C2−C3−C4) ∠(C3−C4−C5) ∠(C4−C5−C6) ∠(C5−C6−N) ∠(C6−N−C2)

2-fluoropyridine

3-fluoropyridine

re

rs

ro

re

rs

ro

re

rs

ro

1.343 1.395 1.393

1.340(2) 1.390(3) 1.394(2)

1.340(5) 1.397(6) 1.394(6)

123.6 118.7 118.3

123.8(3) 118.6(3) 118.3(2)

123.7(5) 118.5(6) 118.5(6)

116.9

116.8(2)

117.1(5)

1.314 1.392 1.389 1.397 1.390 1.347 126.0 116.8 119.0 118.5 123.2 116.4

1.310(18) 1.387(18) 1.386(11) 1.40(4) 1.39(4) 1.345(6) 127(2) 116.5(16) 119(4) 118(4) 123(4) 116.0(15)

1.312(4) 1.393(4) 1.385(5) 1.401(3) 1.390(3) 1.343(4) 126.4(1) 116.5(2) 119.1(1) 118.3(1) 123.4(1) 116.2(1)

1.341 1.390 1.385 1.393 1.394 1.343 121.9 121.1 116.8 119.1 123.4 117.6

1.336(13) 1.38(2) 1.38(2) 1.395(8) 1.39(2) 1.34(2) 122(2) 122(2) 116.4(17) 119.1(19) 123(2) 118(2)

1.336(7) 1.388(5) 1.384(4) 1.395(7) 1.394(4) 1.343(6) 121.8(2) 121.4(2) 116.6(3) 119.0(1) 123.5(1) 117.6(2)

a

Note that while the Kraitchman equations provide only absolute values of the coordinates, the signs are inferred on the basis of the orientation of the conformer in its principal axis system based on the ab initio predictions. bThe parameters in bold were fit to reproduce the rotational constants. Other parameters were calculated from these using trigonometric relationships. cRotational constants from ref 16 used to derived the rs and ro structures including deuterium isotopologues.

length compared with that of C2−C3 which is consistent with our experimental results. The participation of fluorine’s p-electrons in the π-system through this hyperconjugation effect would also explain the observed bond length alternation in 2-fluoropyridine. Deviations in the ro structure of 3-fluoropyridine compared with that of pyridine are less pronounced. The largest changes occur in the bond lengths involving the substituted carbon (C3−C4 and C2−C3) which decrease by 0.010 and 0.009 Å, respectively. The other bond lengths in the ring (including the N−C bonds) vary little compared with those of pyridine with differences of only 0.001−0.004 Å and no obvious bond length alternation. The localization of the structural changes and their symmetry about C3 suggests that the nitrogen atom is too far away to have a significant effect on any π-donation from fluorine. For example, if a hyperconjugation model is invoked, the resonance structures that place the negative charge on C2 versus C4 are not significantly different in energy. The localized structural changes in 3-fluoropyridine may also be explained by a simple inductive argument due to the electron withdrawing nature of fluorine (a known σ-acceptor). If electron density from the pyridine ring is pulled toward C3 to compensate for its increased positive charge, the adjacent bonds would be strengthened. This is also consistent with Bent’s rule in that electron density directed along the C−F bond through increased p-character in one hybrid orbitals will increase the s-character of the remaining hybrid orbitals used to create σ-bonds with C2 and C4. This increased s-character would result in shorter bond lengths for C2−C3 and C3−C4. 14 N Hyperfine Analysis. The 14N nuclear quadrupole constants for the various isotopologues in Tables 1 and 2 show a small degree of variation relative to the parent species as a consequence of changes in the principal inertial axes with heavy atom substitution. The magnitudes and trends of the coupling constants were verified by ab initio calculations of each 13C isotopologue. To better understand how fluorination at the ortho position causes greater structural changes compared to meta substitution, the electronic environments around the nitrogen atoms were compared through analysis of the observed 14N hyperfine structure which was well-resolved for both

density in the region of the C−F bond can be achieved if the hybrid orbital on carbon has increased p-character. This corresponds to a decrease in the p-character of the other two hybrid orbitals centered on carbon. The remaining hybrid orbitals are more s-like which would increase the bond angle at that site of the ring. The same effect was reported for fluorobenzene. The angle at the fluorinated carbon increases by 3.4° according to its microwave spectrum,19 and this value is in good agreement with the theoretical structure.20 The experimentally derived ro structures for 2-fluoropyridine and 3-fluoropyridine are also consistent with ab initio predictions that the pyridine ring geometry is more perturbed in the former compared to the latter.6,8,9 In 2-fluoropyridine, the largest change occurs in the N−C2 bond which is shortened by ∼0.028 Å compared with the ro structure of pyridine. Other bonds in the ring are shortened or lengthened slightly with changes of ∼0.009 Å or less. Interestingly, the other bond involving the substituted carbon (C2−C3) is only slightly shortened (0.006 Å) relative to pyridine suggesting that any perturbation of the π-system by fluorine and its lone pair is more pronounced in the region between C2 and N than toward C3. Furthermore, Boggs and Pang predicted alternating changes in the bond lengths around the 2-fluoropyridine ring when compared with pyridine.6 For example, C2−N (shorter −0.032 Å), N−C6 (longer +0.009 Å), C5−C6 (shorter −0.007 Å), etc., and this matches the trend observed in the ro structures derived here: −0.028 Å, +0.003 Å, and −0.007 Å, respectively. This supports a model in which the π-electron density is more localized along certain bonds in the 2-fluoropyridine ring compared with pyridine which does not exhibit the same bond length alternation. This phenomenon may be explained using Mulliken’s model of hyperconjugation21 as it is well-known that fluorine can behave as a π donor.6 This phenomenon is well established in halogen methane systems as the small size of the fluorine atom is good for orbital overlap with the center carbon.22 If hyperconjugation is present in 2-fluoropyridine, the most favorable resonance structure containing CF+ would put a negative charge on the nitrogen atom. As the added electron density would be preferentially drawn to N (rather than C3), one would expect a larger decrease in the C2−N bond 4086

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Table 4. 14N Quadrupole Coupling Constants (MHz)a and Related Parameters for Pyridine, 2-Fluoropyridine, and 3-Fluoropyridine b

pyridine 2-fluoropyridine 3-fluoropyridine

1.5χaa

0.25(χbb−χcc)a

χaba

θ

χzz

χxx

χyy

∠CNC

αs2

iσ

πc

c−

−7.362 −0.149 −0.052

−0.510 −1.393 −1.742

0.000 2.760 −2.751

0 57.8 61.0

−4.908 −4.477 −4.992

1.434 1.640 1.492

3.474 2.836 3.500

117.1 116.1 117.6

0.313 0.306 0.317

0.22 0.22 0.22

0.07 0.13 0.06

0.51 0.57 0.50

Values from the parent isotopologues were used. bQuadrupole coupling constants from ref 24. θ and ∠CNC were derived from the rotational constants in ref 16. a

put the negative charges on C2 or C4, and thus, the charge on nitrogen would be unaffected. To better visualize the electron densities around nitrogen in pyridine, 2-fluorpyridine, and 3-fluoropyridine, a molecular electrostatic potential (ESP) analysis was performed for each using the cubegen utility in Gaussian03 using the MP2/6-311G++2d2p optimized geometries.12 The three surfaces are shown in Figure 4.

2-fluoropyridine and 3-fluoropyridine. To make this comparison, the experimentally derived nuclear quadrupole coupling constants in the principal inertial axis system (for the parent species) must first be converted to those along the quadrupole coupling tensor axes. As we experimentally determined the offdiagonal term χab, the matrix was first diagonalized using the QDIAG program15 before the χxx, χyy, and χzz constants were determined. The quadrupole coupling constants (in both axis systems) as well as the rotation angles (θ) between the two sets of the coordinates are listed in Table 4. A method for the interpretation of the 14N quadrupole coupling of pyridine is well-described in Gordy and Cook and provides a means to estimate the ionic character across the C− N σ-bonds (iσ) and π-bond (πc), the s-character at nitrogen (αs2), and the negative charge on nitrogen (c−).22 Following their derivation and axis system, the mean ionic character of the C−N bonds can be calculated from

Figure 4. Electrostatic potential surfaces of (a) pyridine, (b) 2fluoropyridine, and (c) 3-fluoropyridine calculated at the MP2 level (6311G++2d2p). The red shading identifies the most electronegative regions.

⎡ χzz = ⎢2(2αs 2) + (1 + iσ) × (1 − 2αs 2) ⎣ −

eQq210(N) ⎤ 1 (2 + iσ + πc)⎥ × ⎦ 1 + (2iσ + πc)ε N 2

χxx − χyy =

eQq210(N) 3 (iσ − πc) × 1 + (2iσ + πc)ε N 2

In displaying the ESP surfaces, we have adopted the convention that the maximum (positive) potential is blue and the minimum (negative) potential is red.23 The ESP surface for 2-fluoropyridine shows a noticeable increase in the electron density shared between N and C2 (as expected from the larger π c value in the analysis of the 14N hyperfine structure above) which is consistent with our observation of substantial shortening of the N−C2 bond. The ESP surface for 3-fluoropyridine is similar to that of pyridine and exhibits only localized changes in electron density near fluorine. This is in agreement with the derived geometry parameters for 3-fluoropyridine which shows little deviation from the structure of pyridine. In conclusion, we observed and assigned the rotational spectra of the normal species of 2-fluoropyridine, 3-fluoropyridine, and their six heavy atom isotopologues involving 13C and 15N substitution. Analysis of the spectra provided estimates of the geometry of the pyridine ring backbone which compare favorably with ab initio results. On the basis of structural calculations, ab initio predictions, and analysis of the 14N hyperfine structure, it is clear that the site of fluorination in the pyridine ring has a profound influence on molecular properties. In particular, fluorine substitution at the ortho position has the largest effect on the electronic structure of the ring as seen through changes in bond lengths and the introduction of bond length alternation about the ring. The derived geometries and electrostatic parameters involving nitrogen are consistent with a model of hyperconjugation.

(1)

(2)

where εN is the charge screening correction (0.30) for a nitrogen p-orbital22 and eQq210 is the atomic orbital coupling of a p-electron on nitrogen (−10 MHz) which has been estimated experimentally and theoretically.22 The s-character αs2 can be estimated from the CNC bond angle by αs 2 =

cos(∠CNC) cos(∠CNC) − 1

(3)

Following the determination of iσ and πc, the negative charge (c−) on nitrogen can be calculated in units of e from c − = 2iσ + πc

(4)

The results for pyridine, 2-fluoropyridine, and 3-fluoropyridine are listed in Table 4 for comparison. The ionic character of the C−N σ-bond (iσ) of the fluorinated pyridines are the same as in pyridine (0.22 e) while the ionic character across the C−N πbond (πc) increases from pyridine (0.07 e) to 2-fluoropyridine (0.13 e) and is almost unchanged in 3-fluoropyridine (0.06 e). A change in the πc character of nitrogen directly affects the total charge c− on the nitrogen atom (eq 4) such that it increases by 0.06 e in 2-fluoropyridine and is virtually unchanged in 3-fluoropyridine. The increased negative charge on nitrogen in 2-fluoropyridine is consistent with the proposed hyperconjugation model which places a negative charge on nitrogen in the energetically favored resonance structure. In the case of 3-fluoropyridine, the two possible resonance structures would

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ASSOCIATED CONTENT

S Supporting Information *

Additional tables. This material is available free of charge via the Internet at http://pubs.acs.org. 4087

dx.doi.org/10.1021/jp301818x | J. Phys. Chem. A 2012, 116, 4082−4088

The Journal of Physical Chemistry A

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (204) 474-8379. Fax: (204) 474-7608. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grants and University Faculty Award programs. We would also like to thank the University of Manitoba Faculty of Science for support awarded to C.W.v.D. in the form of an undergraduate research studentship and our colleagues Dr. H. Luong, Dr. J. Sorensen, and Dr. P. Budzelaar for useful discussions regarding the structures of these molecules.

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REFERENCES

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