Article pubs.acs.org/JPCA
Probing the Electronic Environment of Methylindoles using Internal Rotation and 14N Nuclear Quadrupole Coupling Ranil M. Gurusinghe and Michael J. Tubergen* Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States S Supporting Information *
ABSTRACT: High-resolution rotational spectra were recorded in the 10.5−21.0 GHz frequency range for seven singly methylated indoles. 14N nuclear quadrupole hyperfine structure and spectral splittings arising from tunneling along the internal rotation of the methyl group were resolved for all indole species. The nuclear quadrupole coupling constants were used to characterize the electronic environment of the nitrogen atom, and the program XIAM was used to fit the barrier to internal rotation to the measured transition frequencies. The best fit barriers were found to be 277.1(2), 374.32(4), 414.(5), 331.6(2), 126.8675(15), 121.413(4), and 426(3) cm−1 for 1-methylindole through 7-methylindole, respectively. The fitted barriers were found to be in good agreement with barriers calculated at the ωB97XD/6-311++G(d,p) level. The complete set of experimental barriers is compared to theoretical investigations of the origins of methyl torsional barriers and confirms that the magnitude of these barriers is an overall effect of individual hyperconjugative and structural interactions of many bonding/antibonding orbitals.
■
INTRODUCTION High resolution spectroscopy has been an invaluable tool for studies of large amplitude motions in molecular systems, especially the internal rotation tunneling of methyl groups.1 Fourier transform microwave (FTMW) spectroscopy is capable of achieving the high resolutions needed to resolve small spectral splittings that may arise from significant barriers, and therefore, it has become a key technique for investigating internal rotation in gas-phase molecules. Many molecular systems with one and two methyl internal rotors have been studied by FTMW spectroscopy.2−4 These studies have contributed to our understanding of the effects of hyperconjugation and local steric interactions on torsional barriers. Because methyl rotors are sensitive to the local and nonlocal electronic environment, a systematic study of the barriers to methyl internal rotation at different sites around the perimeter of a conjugated ring system may reveal small variations in electron density. The series of methylindoles, substituted in positions 1 through 7, is an excellent system that provides a spectroscopic probe of the electronic structure of the conjugated ring system. See Figure 1 for indole ring numbering. Methylindoles are aromatic nitrogen-heterocyclic compounds that have high vapor pressures at room temperature.
They have substantial projections of the dipole moment onto the a- and b-inertial axes (0.4−2.3 D), but μc ≈ 0 because of their nearly planar structures. The barrier to methyl internal rotation changes drastically depending on its position on the heterocyclic ring system. A theoretical study by Sinha et al.5 calculated and compared the predominant Lewis and nonLewis contributions to the overall torsional barrier in methylindoles using natural bond orbitals (NBOs). When the methyl group is associated with the pyrrole ring in 1-, 2-, and 3methylindole, non-Lewis delocalization energy, arising primarily from hyperconjugative interactions of bonding−antibonding orbitals, makes large contributions to the barrier. However, in the toluene-like environment of 4-, 5-, 6-, and 7-methylindole, the barrier is primarily determined by the structural/steric Lewis energy contributions. Bickel et al.6 measured photoionization and fluorescence spectra of all seven monomethylindoles in jet-cooled expansions. The ground and excited electronic state potentials were found to be very similar, and the torsional progressions needed to determine the ground state barrier were measured only for 1-, 5-, and 6-methylindole. The experimentally determined barriers for these three indole isomers provided the first evidence that methyl rotors may be highly useful as a probe of the electronic structure of the indole molecule. Later work by Sammeth et al.7 used laser-induced fluorescence spectroscopy to determine the excited-state barriers to methyl internal rotation for 3- and 5-methylindole and complexes of 5methylindole with helium and water, and they were able to estimate a ground-state barrier for internal rotation in 3Received: February 22, 2016 Revised: April 27, 2016
Figure 1. Numbering scheme for the indole ring system. © XXXX American Chemical Society
A
DOI: 10.1021/acs.jpca.6b01794 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 2. Portion of the microwave spectrum of 1-methylindole showing (A) and (E) tunneling state components and nuclear quadrupole hyperfine components of the 514-413 rotational transition.
Table 1. Theoretically Calculated Spectroscopic Parameters for 1- to 7-Methylindole parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A /MHz Ba/MHz Ca/MHz μaa/D μba/D μca/D V3b/cm−1 V6b/cm−1 rotor lengtha/Å
2635.3 1301.8 876.2 2.23 −1.04 0.00 281.5 −28.6 1.449
3722.2 985.8 785.4 2.20 −1.45 0.09 359.2 −26.5 1.494
2592.3 1263.1 853.9 −0.93 1.72 0.12 409.6 −25.8 1.498
2154.4 1479.6 882.1 1.82 0.39 0.04 338.0 −22.4 1.506
3444.3 1029.6 796.6 1.41 1.17 0.09 125.1 −10.4 1.511
3400.9 1037.5 799.0 −0.65 −1.81 0.04 127.1 −12.9 1.510
2130.0 1510.0 888.6 0.51 −2.27 0.10 436.7 −32.3 1.505
a
a
Calculated at MP2/6-311++G(d,p) level. bCalculated at ωB97XD/6-311++G(d,p) level.
■
methylindole using the Levy group’s measurement8 of the 0a1− 1e splitting and observed intensities. Remmers et al.9 used rotationally resolved ultraviolet excitation spectra to investigate methyl internal rotation and internal-overall rotation effects in 3- and 5-methylindole. The measured barriers to methyl rotation were found to be dependent on the symmetry of the electronic density in the bonds adjacent to the rotor; asymmetric electron densities are associated with higher barriers than symmetric electron densities. A frequency analysis of the rotationally resolved spectra was used to determine the methyl-rotor barrier in 3-methylindole (500(40) cm−1; asymmetric electron density) and 5-methylindole (135(6) cm−1; symmetric electron density). We have investigated the microwave spectra of 1methylindole through 7-methylindole in the 10.5−21 GHz frequency range. Because of the very high resolution of cavity Fourier-transform microwave spectrometers, we were able to measure tunneling splittings arising from the methyl rotor and the 14N nuclear quadrupole hyperfine splittings for each of the indole isomers. Three-fold barrier heights and rotor-axis orientations were determined by fitting the frequencies of the A- and E-state rotational transitions for each methylindole. This complete set of methyl rotor data now allows for reliable comparison to the NBO analysis of indole methyl torsional potentials by Sinha et al.5
EXPERIMENT Microwave spectra of the methylindoles were recorded in the 10.5−21 GHz range using a mini-cavity Fourier-transform microwave spectrometer.10−12 The resonant cavity is formed by two 7.5-in. diameter, 30.5 cm spherical radius of curvature aluminum mirrors. The supersonic expansion is nearly coaxial with the cavity resonator axis. Microwave radiation, generated by an Agilent Technologies E8247C PSG CW synthesizer, is coupled into the cavity using an L-shaped antenna and is used to polarize the gas-phase molecules. Molecular emission signals are detected with the same antenna, frequency-reduced using a heterodyne microwave circuit, and digitized using a National Instruments NI 5112 digitizing board. A custom LabVIEW program interfaces the spectrometer with the computer, performs signal averaging, transforms the time domain signal into the frequency domain, and saves the data. Details of the irradiation and heterodyne circuitry can be found in ref 12. The coaxial arrangement of the supersonic expansion splits each transition’s signal into Doppler doublets centered at the rotational transition frequency. The line width of each Doppler component is approximately 13 kHz at full width at halfmaximum, and the digital frequency resolution, as determined by the sampling rate and length of the free induction decay, is 2.4 kHz. A portion of the rotational spectrum of 1-methylindole is shown in Figure 2. 1-, 2-, 3-, and 5-Methylindole were purchased from SigmaAldrich, 4- and 7-methylindole from AK Scientific, and 6B
DOI: 10.1021/acs.jpca.6b01794 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 3. Electrostatic potential mapped onto a 0.20 au electron density isosurface for the optimized structures of 1- to 7-methylindoles, calculated at ωB97XD/6-311++G(d,p) level.
internal rotation in the methylindoles. The potential energy data used to determine the barrier for each methylindole are given in Table S8. The data were parametrized to V3 and V6 terms, and the parameters describing the barrier are given in Table 1 for each methylindole. A potential energy scan for 1methylindole is shown in Figure S1. Because there were no previous microwave spectroscopic studies reported for any of the methylindoles, we began searching for low-J, a- and b-type rotational transitions for each methylindole guided by ab initio predictions. About 150 lines, with J up to 10 and K−1 up to 3, were measured and assigned for each methylindole. The frequencies of all resolved transitions, along with the assignments, are available in the Supporting Information (Tables S9 through S15). All transitions were observed to be split by Doppler doubling, nuclear quadrupole hyperfine effects, and tunneling between the equivalent minima of the methyl internal rotation potential. The Doppler splitting ranged from 50−140 kHz, depending on the frequency, and the ΔF = 1 components of the nitrogen nuclear quadrupole hyperfine splitting resulted in a triplet pattern over about 500 kHz. Typical A−E splittings from internal rotation tunneling ranged from 1 MHz to 1.2 GHz, with the E-state lines lower in frequency than the A-state lines for most of the transitions. The experimental spectrum for the E and A components of the 514−413 rotational transition of 1methylindole is shown in Figure 2. The nuclear quadrupole hyperfine and tunneling splittings are large enough to be easily resolved, but the tunneling splittings are small enough for straightforward assignment. The magnitude of the internal rotation splittings is consistent with modest torsional barriers as predicted by DFT calculations. Final fitting of the assigned transitions for each methylindole was performed using XIAM. The methylindole spectra were fit to a Hamiltonian including rotational constants (A, B, and C), four quartic centrifugal distortion constants (ΔJ, ΔJK, δJ, and δK), nuclear quadrupole coupling constants (χaa, χbb, and χcc), torsional barrier to methyl internal rotation (V3), the torsional rotor axis angles ∠(i, a) and ∠(i, b), and three higher order parameters: Dpi2J, Dpi2K, and Dpi2−. ∠(i, c) was fixed to 90° because of the planarity of the conjugated rings. Iα was fixed to 3.2 amu Å2 and ΔK was fixed to zero for all seven fits. The rotor axis is very nearly coincident with the a-inertial axis of 2methylindole (see Table S2), so the torsional rotor axis angles ∠(i, a) and ∠(i, b) were fixed to values calculated from the ab initio structure. The three higher-order parameters were not included in the final fit for 2-methylindole because they could
methylindole from Chem-Impex International and used without further purification. Each sample was placed in a reservoir nozzle,13 entrained in argon carrier gas, and pulsed into the vacuum chamber using a modified Series-9 General Valve. The backing pressure of argon was 1.5 atm, and the samples were heated to 120−170 °C to increase the vapor pressure. All ab initio calculations were performed with the Gaussian 0914 at Ohio Super Computer Center. Fully optimized structures and electron density distributions were calculated with wave function-based MP2 methods, and the relaxed potential energy scans were performed with a density functional theory approach (ωB97X-D15) along with the 6-311++G(d,p) basis set. In performing relaxed scans for the methyl torsion, the dihedral angle was fixed in 15° steps while the remaining structural parameters were optimized. The spectral fitting programs RRFIT, ZFAP, QUAD2I, and XIAM16 were used throughout this work. Because the Atorsional state transitions followed semirigid rotor patterns, the A-state lines were initially fit using the RRFIT, ZFAP and QUAD2I programs using a Watson A-reduction Hamiltonian.17 Final fittings, which included both A- and E-state transitions, were performed with the XIAM program.
■
RESULTS Table 1 provides the theoretically calculated rotational constants, dipole moment projections, and barriers to methyl internal rotation for each methylindole, and the fully optimized molecular structures are shown in Figure 3. The figure also illustrates the electrostatic potential energy mapped, with common scaling for each methylindole, onto an isosurface with an electron density of 0.20 au. Regions near the nitrogen atom and carbon atoms 4, 5, and 6 are associated with greater negative charge (red); positive charges are localized on the hydrogen atoms (blue) and the remaining carbon atoms have intermediary charges (white). Atomic coordinates of the equilibrium structures, optimized at the MP2/6-311++G(d,p) level, are given in the Supporting Information (Tables S1 through S7). The lowest energy structures all place one of the CH bonds of the methyl group in the same plane as the indole ring and eclipsing the CC bond with greater π-bond character as evaluated by the localized molecular orbital (LMO) calculations of Sinha.5 The ωB97X-D hybrid density functional, with long-range electron−electron exchange corrections and empirical atom−atom dispersion corrections, was found to be very efficient for obtaining relaxed potential energy scans of C
DOI: 10.1021/acs.jpca.6b01794 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Table 2. Experimentally Determined Spectroscopic Parameters for 1- to 7-Methylindole
a
parameter
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
A/MHz B/MHz C/MHz Pcc/u Å2 ΔJ/kHz ΔJK/kHz δJ/kHz δK/kHz Dpi2J/MHz Dpi2K/MHz Dpi2−/MHz χaa/MHz χbb−χcc/MHz V3/cm−1 ∠(i, a)/° ∠(i, b)/° No. A linesa No. E linesa Δνrms/kHz ΔIrms/u Å2
2651.1270(15) 1305.2571(7) 879.7833(4) 1.690 0.041(4) −0.06(2) 0.013(2) 0.11(3) 0.048(4) −0.15(7) 0.021(5) 1.730(9) 5.455(10) 277.1(2) 50.39(13) 39.61(13) 80 72 7.2 1.628
3790.6183(13) 990.2520(3) 789.2286(2) 1.666 0.0105(15) 0.02(2) 0.0012(11) fixed 0.00 fixed 0.00 fixed 0.00 fixed 0.00 1.74(5) 5.25(3) 374.32(4) fixed 0.00 fixed 90.00 80 74 7.2 2.269
2603.724(4) 1268.785(2) 857.8098(2) 1.633 0.048(2) −0.072(7) 0.0156(7) 0.117(11) 0.17(6) −0.5(2) 0.17(6) 1.783(5) 5.209(5) 414.(5) 54.(2) 35.(2) 79 75 2.8 1.947
2164.6596(6) 1484.9238(4) 885.77636(9) 1.629 0.0409(11) fixed 0.00 0.0158(7) 0.053(6) 0.056(5) −0.097(12) 0.040(5) 1.699(5) 4.993(8) 331.6(2) 65.29(7) 24.71(7) 86 88 1.9 1.677
3459.1966(5) 1033.90925(12) 800.17230(5) 1.656 0.0145(5) 0.030(6) 0.0037(2) 0.044(13) −0.0085(2) 0.399(4) −0.0062(2) 1.728(4) 5.056(3) 126.8675(15) 16.671(6) 73.328(6) 95 57 1.4 2.029
3416.4461(9) 1042.1315(2) 802.74921(9) 1.656 0.0035(7) fixed 0.00 fixed 0.00 −0.34(3) −0.0095(4) 0.397(6) −0.0062(5) 1.707(15) 5.066(9) 121.413(4) 18.118(9) 71.882(9) 63 57 2.1 2.159
2141.0265(13) 1515.2908(11) 892.44164(7) 1.638 0.043(3) −0.020(8) 0.0172(9) 0.042(9) 0.17(4) −0.29(6) 0.16(4) 1.698(5) 4.935(5) 426(3) 71.3(6) 18.7(6) 83 87 2.3 1.719
Including hyperfine components.
Table 3. Quadrupole Analysis For 1- To 7-Methylindole
a
parameter
indolea
1-MI
2-MI
3-MI
4-MI
5-MI
6-MI
7-MI
θ/degrees χcc/MHz χparallel/MHz χperpen/MHz iσ πc c−
18.1 −3.379 1.644 1.735 0.24 0.38 0.36
38.0 −3.593 (13) 2.1 (2) 1.5 (2) 0.32 (4) 0.32 (2) 0.50 (6)
19.2 −3.50 (6) 1.757 (1) 1.738 (1) 0.25 (1) 0.36 (1) 0.39 (1)
4.1 −3.496 (7) 1.713 (1) 1.783 (1) 0.24 (1) 0.36 (1) 0.38 (1)
41.6 −3.346 (9)
29.6 −3.392 (5) 1.634 (1) 1.758 (1) 0.23 (1) 0.38 (1) 0.35 (1)
5.9 −3.39 (2) 1.679 (1) 1.707 (1) 0.25 (1) 0.37 (1) 0.37 (1)
3.5 −3.317 (7) 1.618 (2) 1.698 (1) 0.24 (1) 0.39 (1) 0.35 (1)
Suenram et al. (ref 22).
structural parameters determined with XIAM fall within a few percent of the corresponding values obtained using the fitting program BELGI.18 The ab initio rotational constants for these rigid ring systems were very close to the experimental values and provided an excellent starting point for assigning the measured spectra. The experimental A rotational constants were found to be within 2% of the values calculated at the MP2/6-311++G(d,p) level; B and C rotational constants were within 1% of the calculated values. Because the ab initio structure at this level closely represents the experimental structure, (ΔI)rms values range from 1.6 to 2.3 amu Å2 (where ΔI = Ix(obs) − Ix(model) and x = a, b, and c) for each methylindole species, no further structural fitting is necessary. The ab initio structures are essentially planar, except for two of the hydrogen atoms on the methyl group, which are equidistant above and below the plane of the rings. One methyl CH bond eclipses a ring bond in each optimized structure. This configuration leads to very small calculated values for the Pcc planar moment of 1.60 amu Å2. Experimental values of Pcc, given in Table 2, range from 1.63 to 1.69 amu Å2, confirming the planar structure. Pcc values of 1.6 amu Å2 correspond to two hydrogen atoms being 0.891 Å above and below the plane, in good agreement with the c coordinates of the ab initio optimized structures (Tables S1−S7, the coordinates of the atoms that belong to the methyl rotor are in bold).
not be well determined when included. The best-fit parameters are summarized in Table 2 for each methylindole.
■
DISCUSSION The combined principal-axis/Rho-axis approach employed by XIAM has difficulty fitting the tunneling state transitions arising from low-barrier methyl rotors.18−21 The spectroscopic parameters given in Table 2 appear to fit the experimental data quite well since the largest values of Δνrms are about 7 kHz and are comparable to many fits of less complicated rotational spectra measured by FTMW spectroscopy. The fits described in Table 2, with the exception of 2-methylindole, include the higher-order parameters, but most of these quantities have uncertainties larger than 10%. Fits that did not include the higher-order parameters had Δνrms values of about 10 kHz. Also, the uncertainties on the nuclear quadrupole coupling constants are about an order of magnitude larger than would be expected for the spectrum of a rigid monomer. The E-state components were fit less well than those of the A-state, as seen in Tables S9−S15, even though both sets of transitions were fit simultaneously for each methylindole. These observations suggest that nuclear quadrupole coupling constants and the higher order parameters are somewhat correlated with the other internal rotation parameters and that the barriers to methyl rotation in the methylindoles are near the limit of what XIAM can fit. Nonetheless, Kleiner reports that the barrier and D
DOI: 10.1021/acs.jpca.6b01794 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
(414(5) cm−1 ), and the lowest barriers occur for 5methylindole (126.8675(15) cm−1 ) and 6-methylindole (121.413(4) cm−1). Sinha et al.5 calculated individual Lewis and non-Lewis contributions to the potential barrier through NBO analysis at the B3LYP/6-31G(d,p) level. Individual localized molecular orbital (LMO) bond orders were calculated and used to transform the wave function into the localized Lewis structure. The theoretical study evaluates the overall barrier in terms of many individual interactions including individual steric/ structural effects and hyperconjugative delocalization effects from each and every bond. Substitution at the nitrogen distinguishes 1-methylindole from all the other methylindoles. The higher electronegativity of the nitrogen adds additional ionic character to the NH bond and shortens the rotor length by about 0.05 Å. The potential barrier for 1-methylindole, fitted from experimental data using XIAM, is 277.1 (2) cm−1 and agrees very well with ab initio predictions of 283.1 cm−1. In 1-methylindole, the methyl group has two LMO single bonds vicinal to it, and the biggest contributions to its potential barrier are the nonlocal bonding−antibonding interactions of two LMO CC bonds on either side of to the methyl group.5 Local interactions dominate the contributions to the barrier in all the other methylindoles, where there is a LMO single and a double bond adjacent to the rotor.5 For these methylindoles, CC rotor lengths are nearly equal, ranging from 1.49 to 1.51 Å. When methyl substitution is on the pyrrole ring (2- and 3methylindole), local hyperconjugative interactions of the inplane methyl CH bond with the adjacent pyrrole LMO C C bond provide the largest contributions to the potential barrier.5 The experimentally determined barriers of 374.32 (4) cm−1 and 414 (5) cm−1 for 2- and 3-methylindole are within 15 cm−1 of our theoretical predictions. In 4-, 5-, 6-, and 7methylindole, where the methyl substitution is on the benzene ring, small local distortions of the ring during internal rotation cause the largest contributions to the potential energy barrier.5 However, hyperconjugative interactions of out-of-plane methyl CH bonds with the adjacent benzene-ring LMO CC bonds also make small positive contributions in 4- and 7methylindole. The same interactions add small negative contributions (antibarrier effect) in 5- and 6-methylindole, resulting lowest barriers among all seven species. Our barrier measurements clearly illustrate this effect, where 4- and 7methylindole barriers are nearly 210 and 300 cm−1 greater than those for 5- or 6-methylindole.
The nuclear quadrupole coupling constants can be used as a probe of the electronic environment at the quadrupolar nucleus. Suenram et al.22 rotated the measured quadrupole coupling tensor elements for indole into a frame with axes parallel and perpendicular to the NH bond. Following an analysis given in Gordy and Cook,23 Suenram et al. used the quadrupole coupling constants to determine the ionic character of the NH σ bond (iσ(NH) = 0.24), the π bonding character of the nitrogen pz orbital (πc = 0.38), and the amount of negative charge on the nitrogen (c− = 0.36), which were found to be similar to the values determined earlier for pyrrole.23,24 We rotated χaa and χbb for each methylindole into a frame with axes parallel and perpendicular to the NH bond (NC bond for 1-methylindole). The angle of rotation (θ) and the quadrupole coupling components in this new frame are reported in Table 3. The axes transformation is most sensitive to small uncertainties for rotation angles very close to 45°, so the values of χparallel and χperpen could not be reliably determined for 4-methylindole and could only be determined with 10% uncertainty for 1-methylindole. Values of χparallel and χperpen for the other methylindoles were not sensitive to uncertainty of the rotation angle and could be determined to the third decimal place. The quadrupole coupling components in this new frame are reported in Table 3. Notably χcc, which should remain unchanged for all methylindoles, ranged from −3.317 to −3.593 MHz compared to χcc for indole of −3.379 MHz. The variation suggests that the electronic environment of the quadrupolar nitrogen nucleus depends on the position of methyl substitution. The expressions in Gordy and Cook23 were used to determine the electronic environment of the nitrogen nucleus. The ionic character of the NH σ bond, iσ(NH) (or iσ(NC) for 1-methylindole), amount of pi bonding of the pz(ϕ4) orbital, πc, and the negative charge on the nitrogen atom, c−, for each methylindole are also given in Table 3. These values differ the most for 1-methylindole, due to the effect of methyl substitution directly at the nitrogen. iσ(NH) and c− have uniform values for the remaining methylindoles except for 4methylindole, which could not be determined. The barrier to methyl internal rotation may also serve as a measure of electronic environment around the perimeter of the ring system.6 The fit values of the methyl torsional barriers were compared with values from previous studies, where available, in Table 4. The XIAM fitted barriers agree very well with the previous theoretical and fluorescence spectroscopic results. The only exception is for 3MI, where the potential barrier reported by Remmers et al.9 is more than 50 cm−1 higher than the other reported values. The highest barriers are found for 7-methylindole (426(3) cm−1) and 3-methylindole
■
CONCLUSIONS We report the first microwave spectra of seven singly methyl substituted indoles under supersonic molecular beam conditions. Over 1000 transitions were measured and assigned. The XIAM spectral fitting program was employed to simultaneously fit the splittings due to both 14N nuclear quadrupole coupling and methyl internal rotation. Ab initio calculations at MP2/6-311++G(d,p) level were used to model the structures, and the electrostatic potential distribution of each methylindole was mapped onto an electron density isosurface at ωB97XD/6-311++G(d,p) level. Relaxed potential scans at ωB97XD/6-311++G(d,p) level were used to estimate the potential barrier to methyl internal rotation. The 3-fold barriers, obtained from fitting experimental splittings of rotational transitions, were found to range from 121 to 426 cm−1, depending significantly on the position of the
Table 4. Comparison of Potential Barriers, in cm−1, with Previous Studies for 1- to 7-Methylindole
a
molecule
this work
Sinha et al.5
1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI
277.1(2) 374.32(4) 414.(5) 331.6(2) 126.8675(15) 121.413(4) 426(3)
274 348 419 335 109 104 418
other works 282.8a 443.2b, 500(40)c 132.7a, 135(6)c 123.1a
Bickel et al. (ref 6). bSammeth et al. (ref 7). cRemmers et al. (ref 9). E
DOI: 10.1021/acs.jpca.6b01794 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
(11) Suenram, R. D.; Grabow, J.-U.; Zuban, A.; Leonov, I. A Portable, Pulsed-Molecular Beam, Fourier-Transform Microwave Spectrometer Designed for Chemical Analysis. Rev. Sci. Instrum. 1999, 70, 2127− 2135. (12) Conrad, A. R.; Teumelsan, N. H.; Wang, P. E.; Tubergen, M. J. A Spectroscopic and Computational Investigation of the Conformational Structural Changes Induced by Hydrogen Bonding Networks in the Glycidol-Water Complex. J. Phys. Chem. A 2010, 114, 336−342. (13) Lovas, F. J.; Suenram, R. D.; Fraser, G. T.; Gillies, C. W.; Zozom, J. The Microwave Spectrum of Formamide-Water and Formamide-Methanol Complexes. J. Chem. Phys. 1988, 88, 722−729. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2010. (15) Chai, J.-D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom−Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (16) Hartwig, H.; Dreizler, H. The Microwave Spectrum of trans-2,3Dimethyloxirane in Torsional Excited States. Z. Naturforsch., A: Phys. Sci. 1996, 51, 923−932. (17) Watson, J. K. G. Determination of Centrifugal Distortion Coefficients of Asymmetric Top Molecules. J. Chem. Phys. 1967, 46, 1935−1949. (18) Kleiner, I. Asymmetric-Top Molecules Containing One Methyllike Internal Rotor: Methods and Codes for Fitting and Predicting Spectra. J. Mol. Spectrosc. 2010, 260, 1−18. (19) Zhao, Y.; Stahl, W.; Nguyen, H. V. L Ketone Physics − Structure, Conformations, and Dynamics of Methyl Isobutyl Ketone Explored by Microwave Spectroscopy and Quantum Chemical Calculations. Chem. Phys. Lett. 2012, 545, 9−13. (20) Jelisavac, D.; Cortes-Gomez, D. C.; Nguyen, H. V. L; Sutikdja, L. W.; Stahl, W.; Kleiner, I. The Microwave Spectrum of trans Conformer of Ethyl Acetate. J. Mol. Spectrosc. 2009, 257, 111−115. (21) Nguyen, H. V. L; Stahl, W. The Microwave Spectrum of Isopropenyl Acetate − An Asymmetric Molecule with Two Internal Rotors. J. Mol. Spectrosc. 2010, 264, 120−124. (22) Suenram, R. D.; Lovas, F. J.; Fraser, G. T. Microwave Spectrum and 14N Quadrupole Coupling Constants of Indole. J. Mol. Spectrosc. 1988, 127, 472−480. (23) Gordy, W.; Cook, R. L.; Microwave Molecular Spectra, 3rd ed.; John Wiley and Sons, Inc.: New York, 1984. (24) Nygaard, L.; Nielsen, J. T.; Kirchheiner, J.; Maltesen, G.; Rastrup-Andersen, J.; Sorensen, G. O. Microwave Spectra of Isotopic Pyrroles. Molecular Structure, Dipole Moment, and 14N Quadrupole Coupling Constants of Pyrrole. J. Mol. Struct. 1969, 3, 491−506.
methyl substitution on the indole ring. The experimentally barriers were found to support the previous NBO analysis of the origins of methyl torsional barriers in methylindoles.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b01794. Results of the theoretical calculations including Gaussian principal axis coordinates of the optimized structures (MP2/6-311++G(d,p), methyl torsional potential energy data (ωB97XD/6-311++G(d,p)) for 1- through 7methylindole, and graph of potential energy scan for 1methylindole. Frequencies of the assigned rotational transitions including resolved tunneling and nuclear quadrupole hyperfine transitions for 1- through 7methylindole. (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 330-672-2032. Fax: 330-672-3816. E-mail: mtuberge@ kent.edu. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The Ohio Supercomputer Center is acknowledged for a grant of resources. We also thank Prof. Heinrich Maeder, Prof. Sean A Peebles, Prof. Rebecca A Peebles, and Dr. Arun Manna for their valuable discussions.
■
REFERENCES
(1) Spangler, L. H. Structural Information from Methyl Internal Rotation Spectroscopy. Annu. Rev. Phys. Chem. 1997, 48, 481−510. (2) Woermke, S.; Brendel, K.; Andresen, U.; Mader, H. A Molecular Beam Fourier Transform Microwave Study of 2-Methylpyridine and Its Complex with Argon. Mol. Phys. 2004, 102, 1625−1639. (3) Bakri, B.; Demaison, J.; Kleiner, I.; Margules, L.; Mollendal, H.; Petitprez, D.; Wlodarczak, G. Rotational Spectrum, Hyperfine Structure, and Internal Rotation of Methyl Carbamate. J. Mol. Spectrosc. 2002, 215, 312−316. (4) Favero, L. B.; Evangelisti, L.; Feng, G.; Spada, L.; Caminati, W. Conformation and Internal Motions of Dimethyl Sulfate: A Microwave Spectroscopic Study. Chem. Phys. Lett. 2011, 517, 139−143. (5) Sinha, R. K.; Singh, B. P.; Kundu, T. Origin of Threefold Methyl Torsional Potential in Methylindoles. Theor. Chem. Acc. 2008, 121, 59−70. (6) Bickel, G. A.; Leach, G. W.; Demmer, D. R.; Hager, J. W.; Wallace, S. C. The Torsional Spectra of Jet-Cooled Methyl Substituted Indoles in the Ground and First Excited States. J. Chem. Phys. 1988, 88, 1−8. (7) Sammeth, D. M.; Siewert, S. S.; Callis, P. R.; Spangler, L. H. Methyl Rotor Effects in 3- and 5-Methylindole. J. Phys. Chem. 1992, 96, 5771−5778. (8) Wu, Y. R. High Resolution Ultraviolet Spectroscopic Studies of Indole Derivatives and Their Clusters. Ph.D. Dissertation, University of Chicago, 1990. (9) Remmers, K.; Jalviste, E.; Mistrik, I.; Berden, G.; Meerts, W. L. Internal Rotation Effects in the Rotationally Resolved S1(1Lb) ← S0 Origin Bands of 3-Methylindole and 5-Methylindole. J. Chem. Phys. 1998, 108, 8436−8445. (10) Balle, T. J.; Flygare, W. H. Fabry−Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52, 33−45. F
DOI: 10.1021/acs.jpca.6b01794 J. Phys. Chem. A XXXX, XXX, XXX−XXX
