Fourier transform microwave spectra of n-butanol and isobutanol Taigo Uzuyama,1 Yoshiyuki Kawashima,1 and Eizi Hirota2 Kanagawa Institute of Technology1 The Graduate University for Advanced Studies2

Introduction isobutanethiol G-g form G-g’ form T-g form 3 isomers among the 5 expected stable isomers were found. 　Yugo Tanaka, et. al, 　 n-butanethiol f2 q f1 G-g’-g’ T-g-g’ G-g’-g G-t-g G-t-g’ T-t-g T-g-g 7 isomers among the 14 expected stable isomers were found. 　Yugo Tanaka, et. al, 　 None of trans form about the C-S bond for n-butanethiol and isobutanethiol was found.

Our aim X C4 C3 C2 C1 X C4 C3 C2 C1 X= oxygen or sulfur atom In order to obtain information on stable conformers and internal motions of n-butanol and isobutanol comparing with their thiol, we have analyzed their rotational spectra observed by a FTMW spectrometer. Detection, identification, and characterization of the rotational isomers of n-butanol and isobutanol Comparison of experimental results with ab initio calculations Comparison of the spectra observed using Ar and Ne as a carrier gas Comparison of the stability of rotational isomers of butanol and isobutanol with their thiols.

5 expected conformers of isobutanol isobutanol (2-methyl-1-propanol ) f θ trans T-g T-t trans gauche gauche’ θ f gauche G-g G-g’ G-t

14 expected conformers of butanol ① ② O f2 q C2 q C1 f1 T-t-g trans gauche gauche’ ① ② ④ ⑤ ⑥ ⑦ ⑧ ⑨ ⑩ ⑪ ⑫ ⑬ ⑭ ③ T-t-t C4 C3 ③ ④ ⑤ f1 f2 T-g-t T-g-g T-g-g’ ⑥ ⑦ ⑧ G-t-t G-t-g G-t-g’ ⑨ ⑩ ⑪ G-g-t G-g-g G-g-g’ ⑫ ⑬ ⑭ G-g’-t G-g’-g G-g’-g’

Spectra of 7 conformation isomers Spectra of 3 conformation isomers The previous works Conformational study of 1-butanol by the combined use of vibrational spectroscopy Spectra of 7 conformation isomers Ar-Matrix n-butanol IR Keiichi Ohno, et. al, J. Phys. Chem. 1994, 98, 6924. liquid Study of rotational isomerism of iso-butanol by microwave spectroscopy Spectra of 3 conformation isomers iso-butanol MW Abdurakhmanov, A. A, et. Al, 1989, Abstr.

Instrument ： Fourier transform microwave spectrometer 　　　　　　　　　　　　　　　　　　Experimental Instrument ： Fourier transform microwave spectrometer Sample ： isobutanol or n-butanol Backing pressure : 1.0~3.0 atm　　　　 Carrier gas 　　 : Ar or Ne Shots ： 20~50 Frequency region : 7～24 GHz Step： 0.25 MHz Vacuum chamber Mirror (fixed) Molecular beam injection nozzle MW MW Reservoir heater Mirror (mobile) (33~38 ºC) Diffusion pump Rotary pump

Observed rotational spectra of isobutanol diluted with Ar or Ne 7000 Set1　b- and c-type transitions (a-type transitions were not observed) Ar Frequency / MHz Ne 7000 Frequency / MHz

Observed rotational spectra of isobutanol diluted with Ar or Ne Set4: a-type transitions J=2-1 Frequency / MHz Ne Set2 and Set3 and Set4: a-type transitions J=3-2 J=2-1 Frequency / MHz

Assignment of the rotational isomers of isobutanol Experimental set1 set2 set3 set4 A / MHz B / MHz C / MHz  /uÅ2 N(a-type) N(b-type) N(c-type) 7597.2434(1) 3532.70896(5) 2666.25583(4) -20.0321 15 23 7538.8745(22) 3493.5360(13) 2651.4805(9) -21.0943 11 7 24 7538.1991(4) 3505.1130(2) 2639.2895(1) -19.7428 10 4 6231.4353(1) 4000.0768(2) 3196.2945(1) -49.3298 9 12 MP2/6-311G++(d,p) G-t G-g G-g’ T-t T-g a / D b / D c / D DE/ cm-1 7651 3551 2682 -19.90 0.03 1.42 0.92 0.0 7565 3497 2661 -21.37 1.35 0.72 1.24 21.3 7564 3512 2645 -19.63 1.66 0.83 0.80 116.6 6244 4046 3237 -49.72 0.65 0.00 1.65 63.1 6230 3971 3170 -48.95 1.25 1.32 0.78 169.8

Observed rotational spectra of n-butanol diluted with Ar or Ne Set1 Ar Set2 Set3 13000 14000 15000 16000 Frequency / MHz Set1 Ne Set4 Set5 Set6 Set3 Set2 13000 16000 Frequency / MHz 6 sets of the 14 expected isomers were found for n-butanol.

Assignment of rotational isomers of n-butanol 19375.6 19374.8 Frequency / MHz O C3H7 H Tunneling splitting was observed for a-type transitions of the sixth isomer T-t-g. set1 set2 set3 set4 set5 Experimental set6 A / MHz 12467.7496(81) 12304.9926(10) 18658.9682(16) 12530.6861(31) 12326.5009(53) 18715(49) B / MHz 2371.5176(14) 2330.5978(34) 1978.4033(34) 2335.4384(52) 2343.6818(13) 1962.1067(37) C / MHz 2189.4802(14) 2146.2295(31) 1874.1230(26) 2155.1398(51) 2173.7814(12) 1864.4628(37)  / uÅ2 -22.82 -22.44 -12.87 -22.23 -24.15 -13.51 N(a-type) 18 16 12 19 18 15 N(b-type) 21 10 14 － 3 -- N(c-type) 15 5 – 7 5 -- MP2 6-311++G(d,p) T-g-t G-t-g T-t-t T-g-g’ T-g-g T-t-g A / MHz 12487 12240 18711 12652 12324 18520 B / MHz 2394 2357 1983 2338 2350 1965 C / MHz 2202 2157 1877 2155 2181 1866  / uÅ2 -22.10 -21.42 -12.62　 -21.64 -24.45 -13.62 a / D 0.97 1.37 -0.11 -1.78 0.78 1.69 b / D 1.11 1.40 -1.84 0.25 1.30 -0.12 c / D 0.98 0.82 0.00 1.06 -1.09 1.21 DE/ cm-1 291 150 123 66 121

G-g-g’ G-g-g’ G-g-g G-g-t G-g-t G-g-g G-t-t G-t-t G-t-g’ T-g-t G-g’-g Comparison of stability of various rotational isomers for n-butanol and n-butanethiol 732 * For n-butanol, five conformers were of T-form and one was of G-form. G-g-g’ G-g-g’ 1059 570 G-g-g G-g-t 1026 498 The gauche’ conformation about the CC-CO axis is stabilized by a hydrogen bond between the hydroxyl oxygen and the hydrogen of the C(3) position. G-g-t G-g-g 953 352 G-t-t G-t-t 481 347 G-t-g’ T-g-t 344 DE/ cm-1 292 G-g’-g T-t-t 320 favorable 291 G-t-g G-g’-t 237 C1 C1 C2 C2 150 -0.327 T-t-t G-t-g’ 213 2.56 147 C4 C3 C4 C3 G-g’-g’ G-t-g 165 0.015 123 T-g-g’ G-g’-g 134 121 Calculated distance /Å between the H and O atoms. T-t-g T-g-g’ 128 66 T-g-g G-g’-g’ 116 62 natural bond orbital charge distribution G-g’-t T-g-g 96 T-g-t T-t-g * No C-S trans form was found for n-butanethiol. n-butanol n-butanethiol

Interpretation of the calculated results 2.69 C3 C2 2.74 C1 C4 C3 2.60 C3 C1 C1 2.71 2.55 2.61 T-g-g’ T-g-g T-g-t The calculated distances between O and H(C2) and O and H(C3) of the T-g-t form are 2.60 Å and 2.55 Å , which are close to the sum of the van der Waals radii of oxygen and hydrogen atoms; 1.4 Å + 1.2 Å = 2.6 Å. Calculated distance /Å between the H and O atoms.

Comparison of stability of various rotational isomers of isobutanol with isobutanethiol *No C-S trans forms was found for isobutanethiol. 471 / cm-1 T-t form *All the expected gauche forms of isobutanol were found. DE/ cm-1 170 *The preference of the gauche conformation in isobutanol has been explained by the electrostatic interaction between the hydroxyl oxygen and the hydrogens of the methyls in the isopropyl group. / cm-1 277 T-g form / cm-1 G-t form 116 G-g’ form / cm-1 T-g form 126 63 / cm-1 T-t form / cm-1 G-g’ form 21 32 G-g form / cm-1 / cm-1 G-g form / cm-1 / cm-1 G-t form isobutanol isobutanethiol

Interpretation of the calculated results The calculated distances between O and H of the T-t, G-t, and G-g forms are shown below, which are close to the sum of the van der Waals radii of oxygen and hydrogen atoms: that is, 1.4 Å+ 1.2 Å =2.6 Å. G-t form 2.52 T-g form 2.74 2.64 T-t form 2.58 G-g’ form 2.69 G-g form 2.58 : Calculated distance between the H and O atoms. (Å)

K. N. Houk, et. al, J. Am.Chem. Soc., 1993 Interpretation of the calculated results for NBO 0.0152 favorable unfavorable T-t form -0.3276 T-g form -0.3262 0.0159 -0.0003 favorable 0.0074 0.0157 -0.3227 G-t form favorable G-g form -0.3239 0.01667 0.01209 favorable unfavorable G-g’ form -0.3212 0.0098 0.0001 :natural bond orbital charge distribution Only one induced dipole exists in the G-t, G-g, T-g conformer.　In the T-t conformer there are two induced dipoles. The repulsive steric interactions dominate in the T-t conformer. G-g’ conformer has not induced dipole. K. N. Houk, et. al, J. Am.Chem. Soc., 1993 115, 4170

Comparison of stability of various rotational isomers for isobutanol G-t form G-g form T-t form G-g’ form T-g form 0 / cm-1 21 / cm-1 63 / cm-1 116 / cm-1 170 / cm-1 : experimental result Potential energy of the trans form / MP2 6-311G++(d,p) A large tunneling splitting of the T-g form of isobutanol is expected from the potential energy surface calculated by ab initio MO calculation. T-g T-g T-t DE/ cm-1 It is not easy to assign the rotational spectrum of the T-g form. f / degrees

Summary 1. Four rotational isomers of the five were found for isobutanol. 2. A trans forms was detected for isobutanol , in sharp contrast with isobutanethiol , for which none of rotational isomers exits in trans. 3. Considerable differences were found between the spectra using Ar and Ne as a carrier gas. 4. Six rotational isomers of the 14 expected isomers were found for n-butanol. 5. All trans forms of n-butanol were found. 6. It is not easy to assign the rotational transition of the fifth isomer of isobutanol because of large tunneling splitting expected due to OH group.