Microwave spectra of 1- and 2-bromobutane Soohyun Ka, Jihyun Kim, Heesu Jang and Jung Jin Oh Research Institute of Global Environment Sookmyung Women’s University, Korea 22 June 2015

Abstract The rotational spectrum of 1-bromobutane measured by the 480 MHz bandwidth chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer at the Eastern Illinois University. In this paper, the ab initio calculation and the analysis of rotational spectrum were performed, and the properties of gas molecule are reported. 1-bromobutane have five conformers; aa, ag, ga, gg, gg’. The transitions were assigned to three different conformers which are most stable forms; aa, ag, ga. The spectra for the normal isotopic species and 81Br substitution were observed and assigned. The rotational spectrum of 2-bromobutane has been observed in the frequency region 7-18 GHz. 2-bromobutane has the three possible conformers; G+, A, G-. The difference of their energy is very small, so the spectra of all conformers were found in the full range of our spectrum. Consequentially, the rotational constants, nuclear quadrupole constants, and centrifugal distortion constants were determined and the dipole moment of the aa conformer with 79Br were measured. International Symposium on Molecular Spectroscopy

Previous Study [1-BrBu] F. A. Momany, R. A. Bonham, and W. H. McCoy, Electron diffraction determination of the molecular structure of the butyl halides. IV. The structure and conformational analysis of n-butyl bromide in the gas phase, J. Am. Chem. Soc., 85, 3077-3080 (1963) [1-ClBu] E. M. Belloot, JR. and E. Bright Wilson, Free energy of Conformational Changes in Vapors from Low Resolution Microwave Spectroscopic Intensities, J. Mol. Spec., 66, 41-45 (1977) [1-XBu] W. E. Steinmetz, F. Hickernell, and I. K. Mun, The conformational analysis of 1-Haloalkanes by low resolution microwave spectroscopy, J. Mol. Spec., 68, 173-182 (1977) [1-BrBu] K. Aarset, K. Hagen, R. Stølevik, and P. C. Sæbø, Molecular structure and conformational composition of 1-chlorobutane, 1-bromobutane, and 1-iodobutane as determined by gas-phase electron diffraction and ab initio calculations, Struct. Chem., 6(3), 197-205 (1995) [2-BrBu] K. Aarset, K. Hagen, and R. Stølevik, Molecular structures and conformational compositions of 2-chlorobutane and 2-bromobutane; an investigation using gas-phase electron diffraction data and ab initio molecular orbital calculations, J. Mol. Struct., 567-568, 157-165 (2001) [1-BrBu] O. Takahashi, K. Yamasaki, Y. Kohno, K. Ueda, H. Suezawa, and M. Nishio, Origin of the gauche preference of n-propyl halides and related molecules investigated by ab initio MO calculations: Importance of the CH/n hydrogen bond, Chem. Phys. Lett., 440, 64-49 (2007) International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Possible conformers aa ag ga gg’ gg International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Summary of Previous Study This Study Con-former B+C (MHz) Ratio (%) ΔG (kJ/mol) ΔE aa 1799.8(0.9) 36 22.9 1798.7973 1799.9456 0.079 ga 2307(24) 24 37.7 0.5 1.3 2.9 2314.9399 2321.3071 0.096 ag 1994.8(1.2) 21.3 1.93 4.2 - 2004.3014 2.561 gg 16 17.7 2.39 4.6 3.8 2816.7008 2893.6109 gg’ 0.42 11.68 14.0 2979.0515 10.599 Method LRMW (b) ED at RT (a) MP2/ 6-311G(d,p) (d) HF/ 6-31G* (c) ED at 18℃ FTMW 6-311+G (2d,2p) F. A. Momany et al., J. Am. Chem. Soc., 85, 3077-3080 (1963) W. E. Steinmetz et al., J. Mol. Spec., 68, 173-182 (1977) (c) K. Aarset et al., Struct. Chem., 6(3), 197-205 (1995) O. Takahashi et al., Chem. Phys. Lett., 440, 64-49 (2007) International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Hyperfine Transitions + - Nuclear quadrupole moments ΔF = +1 ΔF = 0 ΔF = -1 J=5 J=6 F=15/2 F=13/2 F=11/2 F= 9/2 F= 7/2 F=15/2 F=13/2 F=11/2 F= 9/2 F= 7/2 F=15/2 F=13/2 F=11/2 F= 9/2 F= 7/2 ΔJ=+1 2I+1 components  4 transitions Very strong intensity 2I components  3 transitions 2I-1 components  2 transitions Very weak intensity International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Hyperfine Splitting F→F F→F-1 ΔF = +1 F=15/2 F=13/2 F=11/2 F= 9/2 F= 7/2 F=15/2 F=13/2 F=11/2 F= 9/2 F= 7/2 J=5 J=6 F=15/2 F=13/2 F=11/2 F= 9/2 F= 7/2 4.7 MHz F→F 2I components  3 F→F-1 2I-1 components  2 Very weak intensity 2I+1 components  4 possibilities Very strong intensity International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Spectrum assignment Br isotopic ratio 81Br: 49.463% 79Br: 50.537% (a) ga conformer (b) aa conformer (c) gg conformer 79Br 81Br (d) Experimental (7.5 ~ 18 GHz) by JB95 (a) ga conformer μa=1.621 D, μb=1.257 D, μc=0.216 D a-type & b-type transition assigned (b) aa conformer μa=2.179 D, μb=0.744 D, μc=0.000 D only a-type transition assigned (c) gg conformer μa=1.705 D, μb=1.041 D, μc=0.478 D only a-type transition assigned International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Rotational constants calc: ab initio at MP2/6-311+G(2d,2p) ga aa gg 79Br 81Br A (MHz, calc) 8271.3308 16054.1124 6126.6379 B (MHz, calc) 1210.3063 915.1317 1508.2537 C (MHz, calc) 1111.0008 884.6139 1385.3572 A (MHz) 8196.3730(7) 8183.7297(6) 15770.66(9) 15760.27(1) 6240.618(2) 6235.48(8) B (MHz) 1207.26281(1) 1197.95218(9) 914.89072(6) 907.47581(6) 1463.8266(4) 1452.1707(4) C (MHz) 1107.67707(8) 1099.61264(8) 883.90660(7) 876.95290(6) 1352.8742(4) 1342.6930(4) DJ (kHz) 0.3171(8) 0.3116(6) 0.0634(3) 0.05829(3) 1.5084(3) 1.4786(2) DJK (kHz) -5.639(1) -5.675(9) -3.091(1) -3.004(9) -12.775(2) -13.00(1) d1 (kHz) 0.0511(5) 0.0442(5) 0.278(4) 0.281(3) χaa (MHz) 87.165(9) 73.833(9) 363.04(3) 303.67(3) 157.831(1) 132.69(2) χbb-χcc (MHz) 420.2412(6) 350.2016(6) 184.904(1) 153.212(1) 141.976(5) 117.952(7) χab (MHz) 394.70(4) 329.92(4) -337.810(1) -282.005(2) -336.105(3) -280.1(4) χac (MHz) -72.26(2) -59.81(2) 221.615(3) 186.5(6) χbc (MHz) -87.377(2) -74.04(1) -172.68(8) -144.24(7) N 99(26) 105(28) 86(23) 98(26) 47(14) 35(9) Δνrms (kHz) 3.58 5.12 6.08 5.10 3.90 5.71 κ (Kappa) -0.972 -0.996 -0.955 International Symposium on Molecular Spectroscopy

::: 1-bromobutane ::: Dipole moments of aa conformer Observed (point) and calculated (continuous lines) frequency shifts for some of the observed ΔMF=0 Stark components in CH3CH2CH2CH279Br aa conformer As the aa conformer has symmetry plane, it is expected the aa conformer would possess non-zero μa and μb dipole moment components. aaobs aacalc Δobs-calc μa (D) 2.229(13) 2.179 0.050 μb (D) 0.603(60) 0.744 -0.141 μc (D) 0.000 μtot (D) 2.309 2.303 0.006 N 49 International Symposium on Molecular Spectroscopy

::: 2-bromobutane ::: Structure & ab initio calculation G+ A G- ΔE (kJ/mol) MP2/6-311+G(2d,2p) 0.00 2.08 2.67 Dihedral angle Calculated angle G+ +60° +63.80° A 180° 174.57° G- -60° -67.03° 2-bromobutane ab initio calculation result with MP2/6-311+G(2d,2p) Previous study This study Conformer ΔE (kJ/mol) G+ 0.00 A 1.77 2.08 G- 3.52 2.67 Method HF/6-311+G(d,p) -(a) MP2/6-311+G(2d,2p) (a) K. Aarset, et al., J. Mol. Struct., 567-568, 157-165 (2001) International Symposium on Molecular Spectroscopy

::: 2-bromobutane ::: Spectrum assignment Br isotopic ratio 81Br: 49.463% 79Br: 50.537% (a) G+ conformer (b) A conformer (c) G- conformer (d) Experimental (7.5 ~ 18 GHz) (a) G+ conformer μa=1.910 D μb=1.010 D μc=0.280 D a-type & b-type transition assigned (b) A conformer μa=2.250 D μb=0.440 D μc=0.260 D only a-type transition assigned (c) G- conformer μa=2.010 D μb=0.780 D μc=0.520 D only a-type transition assigned International Symposium on Molecular Spectroscopy

::: 2-bromobutane ::: Rotational constants calc: ab initio at MP2/6-311+G(2d,2p) G+ A G- 79Br 81Br A (MHz, calc) 3882.291333 6534.080423 4595.988187 B (MHz, calc) 2277.22846 1553.409642 1993.058953 C (MHz, calc) 1526.229396 1319.320593 1630.532496 A (MHz) 3857.83420(2) 3853.30685(2) 6412.915(3) 6409.136(2) 4532.801(1) 4530.476(1) B (MHz) 2272.32632(2) 2253.63508(2) 1555.2157(4) 1542.28983(3) 1998.3289(1) 1981.7974(9) C (MHz) 1520.60969(1) 1511.52569(1) 1316.55295(3) 1307.12676(3) 1626.3843(6) 1615.1494(6) ΔJ (kHz) 0.709(4) 0.697(4) 0.1470(3) 0.1576(1) 0.683(1) 0.640(9) ΔJK (kHz) -1.369(1) -1.301(1) 0.47(3) 0.54(3) -1.76(7) -1.68(6) δ1 (kHz) 0.2654(3) 0.2517(3) 0.027(3) 0.0244(2) 0.207(1) 0.197(9) χaa (MHz) 300.1367(2) 253.8573(2) 446.043(9) 372.949(9) 317.134(1) 266.182(1) χbb-χcc (MHz) 135.6128(3) 110.2848(3) 7.632(3) 5.884(3) 62.884(3) 51.616(3) χab (MHz) 321.18(6) 266.64(9) 179.19(4) 149.08(5) -274.8(6) -230.8(7) χac (MHz) -140.93(2) -117.00(2) 145.47(9) 121.79(8) 200.6(9) 165.2(1) χbc (MHz) -82.16(1) -68.25(2) 36.62(3) 33.1(3) -96.21(9) -78.70(2) N 135(25) 127(25) 59(15) 66(17) 35(9) 43(11) Δνrms (kHz) 5.97 6.05 3.99 5.17 3.53 5.33 κ (Kappa) -0.357 -0.366 -0.906 -0.908 -0.744 -0.748 International Symposium on Molecular Spectroscopy

::: 2-bromobutane ::: Hyperfine splitting of G+ isomer G+ conformer Stable, strong intensity F → F+1, F → F component observed & assigned G+ 79Br G+ 81Br Experimental spectrum 404-303 G+ 79Br G+ 81Br 5/2-5/2 13569.05095 -0.01 13503.34265 7/2-7/2 13615.85930 13542.54050 11/2-9/2 13641.61370 0.01 13564.32606 0.00 9/2-7/2 13645.14445 13567.15612 5/2-3/2 13649.35960 13570.93795 7/2-5/2 13652.98260 13573.83347 9/2-9/2 13726.48555 13635.47560 ΔF = 0 ΔF = 0 (Δν = 150 MHz) ΔF = +1 (Δν = 10 MHz) ΔF = 0 International Symposium on Molecular Spectroscopy

::: 2-bromobutane ::: Dipole moments of G+ conformer Observed (point) and calculated (continuous lines) frequency shifts for some of the observed ΔMF=0 Stark components in CH3CH79BrCH2CH3 G+ conformer N is the number of distinct Stark measurement G+obs G+calc Δobs-calc μa (D) 1.771(16) 1.910 -0.139 μb (D) 1.180(29) 1.010 0.170 μc (D) 0.412(17) 0.280 0.132 μtot (D) 2.168 2.180 -0.012 N 52 International Symposium on Molecular Spectroscopy

CONCLUSION The ab initio calculation and the measurement of the rotational spectra of 1- and 2-bromobutane were performed. From the rotational spectrum analysis, the most stable conformer is determined to be the ga conformer for the 1-bromobutane in this study. Our experimental results are consistent with one of the previous studies that the gauche conformer is most stable. The reason is that the distances between the bromine and the closest hydrogen in the ga, gg and gg’ conformer (2.92~2.94 Å) are remarkably shorter than van der waals distance (3.05 Å). For the 2-bromobutane, the G+ isomer is the most stable and it is consistent with ab initio calculation. The measured dipole moments of 1- and 2-bromobutane show good agreement with the ab initio calculation. International Symposium on Molecular Spectroscopy

Acknowledgement Dr. Prof. Sean A. Peebles in Eastern Illinois University Dr. Prof. Rebecca A. Peebles in Eastern Illinois University This research was supported by Basic Science Research Program funded by the Ministry of Science, ICT and future Planning. Photos from Dr. Prof. Sean A. Peebles Lab in EIU http://www.ux1.eiu.edu/~cfsap/ International Symposium on Molecular Spectroscopy