Methylindoles – Microwave Spectroscopy 1-7 methyl substituted Indoles What can we learn about the electronic environment of the methylindoles using internal rotation and quadrupole coupling? Continuation of previous work, presented at ISMS 2014 – RJ03 MW spectroscopy of 1-,2-,3- and 5-methylindoles RANIL M. GURUSINGHE, MICHAEL TUBERGEN Department of Chemistry, Kent State University, Ohio

Methods Methylindoles – Microwave Spectroscopic Study Recorded and assigned the rotational spectra of 1-7 methylindoles Determined 14N quadrupole coupling constants (χaa, χbb, χcc) Determined the potential barriers to methyl internal-rotation (V3) Computational calculations at MP2 and DFT level: rotational constants potential barriers Compared the χaa, χbb, χcc, and V3 Fabry–Pérot cavity based FT-Microwave Spectrometer at Kent State University 7.5” diameter mirrors . 10.5 – 21 GHz 2.4 kHz resolution Doppler splitting . 13 kHz FWHM of each component Reservoir nozzle . Can be heated up to 200 °C Methylindoles – Microwave Spectroscopic Study

Methylindoles – Microwave Spectroscopic Study MW Spectra First MW spectroscopic investigation on 1-7 methylindoles methyl internal rotation splitting 14N quadrupole splitting Methylindoles – Microwave Spectroscopic Study

Methylindoles – Microwave Spectroscopic Study V3 Barrier parameter 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI A/MHz 2651.1270(15) 3790.6183(13) 2603.724(4) 2164.6596(6) 3459.1966(5) 3416.4461(9) 2141.0265(13) B/MHz 1305.2571(7) 990.2520(3) 1268.785(2) 1484.9238(4) 1033.90925(12) 1042.1315(2) 1515.2908(11) C/MHz 879.7833(4) 789.2286(2) 857.8098(2) 885.77636(9) 800.17230(5) 802.74921(9) 892.44164(7) Pcc/u Å2 1.690 1.666 1.633 1.629 1.656 1.638 ΔJ/kHz 0.041(4) 0.0105(15) 0.048(2) 0.0409(11) 0.0145(5) 0.0035(7) 0.043(3) ΔJK/kHz -0.06(2) 0.02(2) -0.072(7) fixed 0.00 0.030(6) -0.020(8) δJ/kHz 0.013(2) 0.0012(11) 0.0156(7) 0.0158(7) 0.0037(2) 0.0172(9) δK/kHz 0.11(3) 0.117(11) 0.053(6) 0.044(13) -0.34(3) 0.042(9) Dpi2J/MHz 0.048(4) 0.17(6) 0.056(5) -0.0085(2) -0.0095(4) 0.17(4) Dpi2K/MHz -0.15(7) -0.5(2) -0.097(12) 0.399(4) 0.397(6) -0.29(6) Dpi2-/MHz 0.021(5) 0.040(5) -0.0062(2) -0.0062(5) 0.16(4) χaa/MHz 1.730(9) 1.74(5) 1.783(5) 1.699(5) 1.728(4) 1.707(15) 1.698(5) χbb-χcc/MHz 5.455(10) 5.25(3) 5.209(5) 4.993(8) 5.056(3) 5.066(9) 4.935(5) V3/cm-1 277.1(2) 374.32(4) 414.(5) 331.6(2) 126.8675(15) 121.413(4) 426(3) ∠(i,a)/º 50.39(13) 54.(2) 65.29(7) 16.671(6) 18.118(9) 71.3(6) ∠(i,b)/º 39.61(13) fixed 90.00 35.(2) 24.71(7) 73.328(6) 71.882(9) 18.7(6) No. A lines 80 79 86 95 63 83 No. E lines 72 74 75 88 57 87 Δνrms/kHz 7.2 2.8 1.9 1.4 2.1 2.3 a-including the hyperfine components Methylindoles – Microwave Spectroscopic Study

Quality of the XIAM Fits parameter 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI A/MHz 2651.1270(15) 3790.6183(13) 2603.724(4) 2164.6596(6) 3459.1966(5) 3416.4461(9) 2141.0265(13) B/MHz 1305.2571(7) 990.2520(3) 1268.785(2) 1484.9238(4) 1033.90925(12) 1042.1315(2) 1515.2908(11) C/MHz 879.7833(4) 789.2286(2) 857.8098(2) 885.77636(9) 800.17230(5) 802.74921(9) 892.44164(7) Pcc/u Å2 1.690 1.666 1.633 1.629 1.656 1.638 ΔJ/kHz 0.041(4) 0.0105(15) 0.048(2) 0.0409(11) 0.0145(5) 0.0035(7) 0.043(3) ΔJK/kHz -0.06(2) 0.02(2) -0.072(7) fixed 0.00 0.030(6) -0.020(8) δJ/kHz 0.013(2) 0.0012(11) 0.0156(7) 0.0158(7) 0.0037(2) 0.0172(9) δK/kHz 0.11(3) 0.117(11) 0.053(6) 0.044(13) -0.34(3) 0.042(9) Dpi2J/MHz 0.048(4) 0.17(6) 0.056(5) -0.0085(2) -0.0095(4) 0.17(4) Dpi2K/MHz -0.15(7) -0.5(2) -0.097(12) 0.399(4) 0.397(6) -0.29(6) Dpi2-/MHz 0.021(5) 0.040(5) -0.0062(2) -0.0062(5) 0.16(4) χaa/MHz 1.730(9) 1.74(5) 1.783(5) 1.699(5) 1.728(4) 1.707(15) 1.698(5) χbb-χcc/MHz 5.455(10) 5.25(3) 5.209(5) 4.993(8) 5.056(3) 5.066(9) 4.935(5) V3/cm-1 277.1(2) 374.32(4) 414.(5) 331.6(2) 126.8675(15) 121.413(4) 426(3) ∠(i,a)/º 50.39(13) 54.(2) 65.29(7) 16.671(6) 18.118(9) 71.3(6) ∠(i,b)/º 39.61(13) fixed 90.00 35.(2) 24.71(7) 73.328(6) 71.882(9) 18.7(6) No. A lines 80 79 86 95 63 83 No. E lines 72 74 75 88 57 87 Δνrms/kHz 7.2 2.8 1.9 1.4 2.1 2.3 a-including the hyperfine components Methylindoles – Microwave Spectroscopic Study

Quality of the XIAM Fits parameter 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI A/MHz 2651.1270(15) 3790.6183(13) 2603.724(4) 2164.6596(6) 3459.1966(5) 3416.4461(9) 2141.0265(13) B/MHz 1305.2571(7) 990.2520(3) 1268.785(2) 1484.9238(4) 1033.90925(12) 1042.1315(2) 1515.2908(11) C/MHz 879.7833(4) 789.2286(2) 857.8098(2) 885.77636(9) 800.17230(5) 802.74921(9) 892.44164(7) Pcc/u Å2 1.690 1.666 1.633 1.629 1.656 1.638 ΔJ/kHz 0.041(4) 0.0105(15) 0.048(2) 0.0409(11) 0.0145(5) 0.0035(7) 0.043(3) ΔJK/kHz -0.06(2) 0.02(2) -0.072(7) fixed 0.00 0.030(6) -0.020(8) δJ/kHz 0.013(2) 0.0012(11) 0.0156(7) 0.0158(7) 0.0037(2) 0.0172(9) δK/kHz 0.11(3) 0.117(11) 0.053(6) 0.044(13) -0.34(3) 0.042(9) Dpi2J/MHz 0.048(4) 0.17(6) 0.056(5) -0.0085(2) -0.0095(4) 0.17(4) Dpi2K/MHz -0.15(7) -0.5(2) -0.097(12) 0.399(4) 0.397(6) -0.29(6) Dpi2-/MHz 0.021(5) 0.040(5) -0.0062(2) -0.0062(5) 0.16(4) χaa/MHz 1.730(9) 1.74(5) 1.783(5) 1.699(5) 1.728(4) 1.707(15) 1.698(5) χbb-χcc/MHz 5.455(10) 5.25(3) 5.209(5) 4.993(8) 5.056(3) 5.066(9) 4.935(5) V3/cm-1 277.1(2) 374.32(4) 414.(5) 331.6(2) 126.8675(15) 121.413(4) 426(3) ∠(i,a)/º 50.39(13) 54.(2) 65.29(7) 16.671(6) 18.118(9) 71.3(6) ∠(i,b)/º 39.61(13) fixed 90.00 35.(2) 24.71(7) 73.328(6) 71.882(9) 18.7(6) No. A lines 80 79 86 95 63 83 No. E lines 72 74 75 88 57 87 Δνrms/kHz 7.2 2.8 1.9 1.4 2.1 2.3 a-including the hyperfine components Methylindoles – Microwave Spectroscopic Study

Theoretical Calculations Electrostatic potential mapped onto a 0.20 a.u. electron density isosurface for the optimized structures, calculated at MP2/6-311++G(d,p) level 1-methylindole 2-methylindole 3-methylindole 2 measurable dipole moment rotor length? 4-methylindole 5-methylindole 6-methylindole 7-methylindole Methylindoles – Microwave Spectroscopic Study

Structure from Rotational Constants parameter 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI A/MHz 2651.1270(15) 3790.6183(13) 2603.724(4) 2164.6596(6) 3459.1966(5) 3416.4461(9) 2141.0265(13) B/MHz 1305.2571(7) 990.2520(3) 1268.785(2) 1484.9238(4) 1033.90925(12) 1042.1315(2) 1515.2908(11) C/MHz 879.7833(4) 789.2286(2) 857.8098(2) 885.77636(9) 800.17230(5) 802.74921(9) 892.44164(7) Pcc/u Å2 1.690 1.666 1.633 1.629 1.656 1.638 ΔJ/kHz 0.041(4) 0.0105(15) 0.048(2) 0.0409(11) 0.0145(5) 0.0035(7) 0.043(3) ΔJK/kHz -0.06(2) 0.02(2) -0.072(7) fixed 0.00 0.030(6) -0.020(8) δJ/kHz 0.013(2) 0.0012(11) 0.0156(7) 0.0158(7) 0.0037(2) 0.0172(9) δK/kHz 0.11(3) 0.117(11) 0.053(6) 0.044(13) -0.34(3) 0.042(9) Dpi2J/MHz 0.048(4) 0.17(6) 0.056(5) -0.0085(2) -0.0095(4) 0.17(4) Dpi2K/MHz -0.15(7) -0.5(2) -0.097(12) 0.399(4) 0.397(6) -0.29(6) Dpi2-/MHz 0.021(5) 0.040(5) -0.0062(2) -0.0062(5) 0.16(4) χaa/MHz 1.730(9) 1.74(5) 1.783(5) 1.699(5) 1.728(4) 1.707(15) 1.698(5) χbb-χcc/MHz 5.455(10) 5.25(3) 5.209(5) 4.993(8) 5.056(3) 5.066(9) 4.935(5) V3/cm-1 277.1(2) 374.32(4) 414.(5) 331.6(2) 126.8675(15) 121.413(4) 426(3) ∠(i,a)/º 50.39(13) 54.(2) 65.29(7) 16.671(6) 18.118(9) 71.3(6) ∠(i,b)/º 39.61(13) fixed 90.00 35.(2) 24.71(7) 73.328(6) 71.882(9) 18.7(6) No. A linesa 80 79 86 95 63 83 No. E linesa 72 74 75 88 57 87 Δνrms/kHz 7.2 2.8 1.9 1.4 2.1 2.3 ΔIrms/u Å2 1.628 2.269 1.947 1.677 2.029 2.159 1.719 Methylindoles – Microwave Spectroscopic Study

Methylindoles – Microwave Spectroscopic Study Quadrupole Analysis parameter Indolea 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI χaa/MHz 1.726 1.730(9) 1.74(5) 1.783(5) 1.699(5) 1.728(4) 1.707(15) 1.698(5) χbb-χcc/MHz 5.031 5.455(10) 5.25(3) 5.209(5) 4.993(8) 5.056(3) 5.066(9) 4.935(5) θ/ degrees 18.1 38.0 19.2 4.1 41.6 29.6 5.9 3.5 χcc / MHz -3.379 -3.593 (13) -3.50 (6) -3.496 (7) -3.346 (9) -3.392 (5) -3.39 (2) -3.317 (7) χparallel / MHz 1.644 2.1 (2) 1.757 (1) 1.713 (1) - 1.634 (1) 1.679 (1) 1.618 (2) χperpen / MHz 1.735 1.5 (2) 1.738 (1) 1.783 (1) 1.758 (1) 1.707 (1) 1.698 (1) iσ (N-H) 0.24 0.32 (4) 0.25 (1) 0.24 (1) 0.23 (1) πc (Pz) 0.38 0.32 (2) 0.36 (1) 0.38 (1) 0.37 (1) 0.39 (1) c- (N) 0.36 0.50 (6) 0.35 (1) a Suenram, R. D.; Lovas, F. J.; Fraser, G. T. Microwave Spectrum and 14N Quadrupole Coupling Constants of Indole. J. Mol. Spectrosc. 1998, 127, 472-480. Methylindoles – Microwave Spectroscopic Study

Methylindoles – Microwave Spectroscopic Study Quadrupole Analysis parameter Indolea 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI χaa/MHz 1.726 1.730(9) 1.74(5) 1.783(5) 1.699(5) 1.728(4) 1.707(15) 1.698(5) χbb-χcc/MHz 5.031 5.455(10) 5.25(3) 5.209(5) 4.993(8) 5.056(3) 5.066(9) 4.935(5) θ/ degrees 18.1 38.0 19.2 4.1 41.6 29.6 5.9 3.5 χcc / MHz -3.379 -3.593 (13) -3.50 (6) -3.496 (7) -3.346 (9) -3.392 (5) -3.39 (2) -3.317 (7) χparallel / MHz 1.644 2.1 (2) 1.757 (1) 1.713 (1) - 1.634 (1) 1.679 (1) 1.618 (2) χperpen / MHz 1.735 1.5 (2) 1.738 (1) 1.783 (1) 1.758 (1) 1.707 (1) 1.698 (1) iσ (N-H) 0.24 0.32 (4) 0.25 (1) 0.24 (1) 0.23 (1) πc (Pz) 0.38 0.32 (2) 0.36 (1) 0.38 (1) 0.37 (1) 0.39 (1) c- (N) 0.36 0.50 (6) 0.35 (1) a Suenram, R. D.; Lovas, F. J.; Fraser, G. T. Microwave Spectrum and 14N Quadrupole Coupling Constants of Indole. J. Mol. Spectrosc. 1998, 127, 472-480. Methylindoles – Microwave Spectroscopic Study

Barrier to Methyl Internal Rotation molecule this work fitted barrier this work theoretical barrier ωB97XD/6-311++G(d,p) Sinha et al.5 B3LYP/6-31G(d,p) other experimental works V3 (cm-1) V6 (cm-1) 1-MI 277.1(2) 281.5 -28.6 274 282.8a 2-MI 374.32(4) 359.2 -26.5 348 - 3-MI 414.(5) 409.2 -25.8 419 443.2b, 500(40)c 4-MI 331.6(2) 338.0 -22.4 335 5-MI 126.8675(15) 125.1 -10.4 109 132.7a, 135(6)c 6-MI 121.413(4) 127.1 -12.9 104 123.1a 7-MI 426(3) 436.7 -32.3 418 a 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. 1987, 88, 1-8. b 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. c 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. Methylindole Electronic Environment – Microwave Spectroscopic Study

Barrier Forming Factors NBO analysis – Sinha, R. K.; Singh, B. P.; Kundu, T. Origin of Threefold Methyl Torsional Potential in Methylindoles. Theor. Chem. Acc. 2008, 121, 59-70. 1-MI non-local hyperconjugative interactions 2-MI, 3-MI local hyperconjugative interactions 4-MI, 5-MI, 6-MI, 7-MI local structural distortions Methylindoles – Microwave Spectroscopic Study

Methylindoles – Microwave Spectroscopic Study Acknowledgement Prof. Heinrich Maeder Prof. Sean Peebles Prof. Rebecca Peebles Ohio Supercomputer Center Kent State University Methylindoles – Microwave Spectroscopic Study

Theoretical Calculations parameter 1-MI 2-MI 3-MI 4-MI 5-MI 6-MI 7-MI A/MHza 2635.3 3722.2 2592.3 2154.4 3444.3 3400.9 2130.0 B/MHza 1301.8 985.8 1263.1 1479.6 1029.6 1037.5 1510.0 C/MHza 876.2 785.4 853.9 882.1 796.6 799.0 888.6 μa/Da 2.23 2.20 -0.93 1.82 1.41 -0.65 0.51 μb/Da -1.04 -1.45 1.72 0.39 1.17 -1.81 -2.27 μc/Da 0.00 0.09 0.12 0.04 0.10 V3/cm-1 b 281.5 359.2 409.6 338.0 125.1 127.1 436.7 V6/cm-1 b -28.6 -26.5 -25.8 -22.4 -10.4 -12.9 -32.3 rotor length/Åa 1.449 1.494 1.498 1.506 1.511 1.510 1.505 Methylindoles – Microwave Spectroscopic Study