J. CHEM. SOC. FARADAY TRANS., 1994, 90(19), 2849-2856 2849

Microwave Spectrum and Conformation of 5,6=Dihydro=2H=thiopyran

Luis A. Leal, David G. Lister and Jose L. Alonso* Departmento de Quimica-Fisica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain Mary M. J. Tecklenburg, John R. Villarreal and Jaan Laane Department of Chemistry, Texas A& M University, College Station, TX 77848, USA

The microwave spectrum of 5,6-dihydro-W-thiopyran has been observed between 12.4 and 40 GHz. Rotational and quartic centrifugal distortion constants have been obtained for t h e ground and seven excited vibrational states. The fundamental wavenumbers of t h e two lowest vibrations have been obtained from relative intensity measurements a s v36 = 134 (12) cm- ' and v35 = 219 (18) cm-'. The electric dipole moment of the ground vibra- tional state has been determined from Stark effect measurements a s [in D, 1 D (Debye) z 3.33564 x C m], pa = 1.35 ( l ) , pb < 0.1, pc = 0.74 (2), with a total dipole moment of p = 1.54 (2). The rotational constants and relatively large pc component of t h e electric dipole moment are consistent with 5,6-dihydro-W-thiopyran having a twisted or half-chair conformation. A molecular structure has been derived from the ground vibrational state rotational constants and planar moments of inertia using t h e method of predicate observations. The equilibrium twist angle of t h e ring is determined as z = 32.5 (1.5)'.

Introduction Cyclohexene and a number of six-membered ring molecules related to it by the replacement of methylene groups by oxygen atoms have a twisted or half-chair (I) rather than a bent (11) or planar ring equilibrium conformation.'

11 111

Recently, the conformation of a sulfur-substituted analogue of cyclohexene, 5,6-dihydro-2H-thiopyran (111), has been determined from its vibrational spectrum and molecular mechanics computations;2 it has also been found to have a twisted equilibrium conformation. In this paper we present a study of the microwave spectrum of 5,6-dihydro-2H-thiopy- ran with the aim of making another determination of its molecular conformation. Such a study would be expected to give further information on the lowest-wavenumber vibra- tions of the molecule.

Experimental The sample of 5,6-dihydro-2H-thiopyran was that used in the study of its vibrational spectrum.2 Microwave spectra were observed using a computer-controlled 33 kHz Stark modula- tion ~pectrometer.~ The microwave absorption cell was main- tained at a temperature of 250 K, with sample pressures of 15-25 mTorr. Frequency measurements are estimated to be accurate to 0.05 MHz. Radiofrequency-microwave double r e ~ o n a n c e ~ . ~ was used in the initial assignment of the spec- trum. The wavenumbers of the excited vibrational states were obtained from relative intensity measurements made using the method of Esbitt and Wilson.6 The electric dipole moment of the ground vibrational state was obtained from Stark effect measurements on the nine components of three J = 3 +- 2 transitions which occur in the frequency range 12.4-18 GHz. The square-wave Stark modulation was super- imposed on the dc voltage from a Fluke 415 high-voltage power supply. The J = 1 + 0 transition of carbonyl sulfide with an electric dipole moment7 of 0.71521 D was used for calibration purposes.

Analysis of the Spectrum A molecular model of 5,6-dihydro-2H-thiopyran (see below) showed that both bent and twisted conformers of the mol- ecule were expected to be very asymmetric oblate rotors with Ray's asymmetry parameter K 0.1. Bond dipole calcu- lations in which only the C-S bonds, with a bond dipole moment of 1.0 D, were assumed to be polar indicated that the main electric dipole component would lie along the a inertial axis for both types of conformer. The initial assign- ment of the pa Q-branch series of lines was made using radiofrequency-microwave double resonance. After this, some weaker pa and p, R-branch lines were assigned. Measure- ments were made up to J = 18 in order to be able to deter- mine the quartic centrifugal distortion constants. The line frequencies were fitted using Watson's A reduced semirigid rotor Hamiltonian* and 111' axis representation. The spectra of seven of the most intense excited vibrational states were also assigned. Measurements were also made up to J = 18 and the line frequencies were fitted in the same way as for the ground vibrational state. The measured line fre- quencies are given in Table 1 and the derived rotational and quartic centrifugal constants, details of the least-squares fits and the vibrational wavenumbers of some of the vibrational states are given in Table 2. Entries for the centrifugal distor- tion constants, S , and S , , without standard errors in Table 1 mean that these have been constrained to the values of the ground vibrational state constants in the least-squares fits. The assignment of the excited vibrational states given in Table 2 is discussed in detail below.

Electric Dipole Moment The frequency displacements of the observed Stark com- ponents were found to be proportional to the square of the electric field and therefore second-order perturbation theoryg was used to derive the principal inertial axis components of the electric dipole moment. The pc component was treated in the same way as pa and p b . This will be justified in the Dis- cussion (below). Several fits were made to the observed Stark coefficients and from these it became apparent that p b was small and not particularly well determined. The small value of & is consistent with our inability to observe transitions due to this electric dipole component even though these were

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2850 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90

Table 1 vibrational states of 5,6-dihydro-2H-thiopyran

Measured line frequencies and differences between the observed and calculated line frequencies (in MHz) for the ground and excited

0 36, 36, 363 transition

obs obs - calc obs obs - calc obs obs - calc obs obs - calc J; , ' K + ,' ' Jk- ~ " K + I "

4 1 . 3 ' 3 0 . 3

4 2 , Z ' 3 1 . 2

4 2 . 3 ' 3 1 . 3

4 3 . 1 4 - 3 2 , l

4 3 . 2 ' 3 2 , 2

5 1 . 4 ' 4 0 . 4

5 2 . 3 ' 4 1 . 3

5 2 . 3 4 - 4 2 . 2

5 2 . 4 ' 4 1 . 4

5 3 . 2 4 - 4 2 . 2

5 3 . 2 ' 4 3 . 1

5 3 , 3 ' 4 2 , 3

5 3 , 3 ' 4 3 , 2

5 4 . 1 4 - 4 3. 1

5 4 . 1 ' 4 4 . 0

5 4 . 2 ' 4 3, 2

5 4 . 2 4 - 4 4 , l

6 1 . 5 ' 5 1 . 4

6 2 . 4 ' 5 2 - 3

6 2 . 5 4 - 5 2 . 4

6 3 . 3 ' 5 3 . 2

6 3 . 4 ' 5 3 . 3

6 4 . 2 ' 5 4 . 1

7 0 - 7 ' 0, 6

l . 6 + 1 . 5

1 . 7 ' 1 . 6

7 2 . 5 4 - 6 2 . 4

7 3 . 5 ' 6 3 . 4

7 4 . 4 4 - 6 4 . 3

8 0 . 8 4 - 7 0. 7

2 . 6 ' 2 . 5

1 . 7 ' 1 . 6

l , 8 ' 1 . 7

2 . 7 ' 2 . 6

6 . 2 ' 7 5, 2

6 . 3 ' 5 . 3

7 . 1 ' 6 . 1

8 7 . 2 ' 6 , 2

9 0,9' 8 0, 8

10 0 . 1 0 + 10 2, 9

10 1 . 1 0 + 10 1. 9

11 0 . 1 1 4- 11 2 . 1 0

11 1 . 1 1 ' 11 1 , l O

14 3 , l l ' 14 5 . 1 0

14 4 . 1 1 + 14 4 . 1 0

14 5 . 1 0 + 14 5, 9

1 . 9 ' 1 . 8

l3 3 . 1 0 ' l3 5 , 9

l4 4 . 1 0 l4 6, 9

l5 0 . 1 5 ' l4 0 . 1 4

l5 1 . 1 5 ' l4 1 . 1 4

l5 3 . 1 2 ' l5 5 . 1 1

l5 4 . 1 1 ' l5 6 . 1 0

l5 4 . 1 2 l5 4 . 1 1

l5 12, 3 4- l5 13. 3

l5 12, 4 ' l5 13. 2

l6 4 . 1 2 ' l6 6 . 1 1

l6 5 . 1 1 4- l6 7 . 1 0

l6 5 , 1 2 ' l6 5 . 1 1

l6 6 . 1 1 ' l6 6 . 1 0

l7 5 . 1 2 ' l7 7 . 1 1

l7 6 . 1 2 ' l7 6 . 1 1

17 7 . 1 1 + 17 7 . 1 0

l7 13, 4 ' l7 14. 4

l7 13 , 5 + l7 14, 3

l8 5 . 1 3 ' l8 7 . 1 2

l8 6 . 1 2 ' l8 8 . 1 1

l8 7 . 1 2 l8 7 . 1 1

18 13, 5 4- l8 14, 5

l8 13. 6 + l8 1 4 , 4

28 665.86 27 799.99 29 239.36 28 984.88 30 121.66 36 205.96 35 142.30 28 776.87 36 389.36 34 917.02 29 5 14.49 36 652.79 26 972.98 37 322.90 28 156.65 38 116.33 27 624.43 29 145.14 33 032.93 28 989.94 35 512.21 31 740.18 34 810.00 29 762.74 33 091.86 29 761.84 36 766.62 33 050.89 36 154.99 38 326.79 33 778.25 37 086.09 33 778.25 37 076.44 59 62 1.59 60 272.09 62 968.24 63 01 8.18 37 794.32 37 794.32 3 1 096.59 3 1 096.59 34 393.03 34 393.03 30 167.33 33 521.81 29 687.33 33 513.26 29 546.56 61 890.57 61 890.57 36 867.57 33 067.57 36 865.48 35 578.89 35 581.60 36 447.67 32 522.64 36 437.41 32 370.32 35 934.73 35 891.24 31 412.44 38 007.84 38 013.05 39 341.99 35 328.44 35 168.03 37 326.05 37 354.56

0.02 - 0.02 0.00 0.04 0.00 0.04

-0.01 0.02 0.01

- 0.01 - 0.02 0.02

- 0.04 0.02

- 0.02 - 0.00 - 0.03 - 0.01 0.02 0.0 1 0.04 0.03

- 0.02 0.05

- 0.02 - 0.05 0.01 0.03 0.02

- 0.02 - 0.07 0.03 0.08

- 0.03 - 0.04 - 0.03 - 0.02 - 0.02 - 0.00 0.02 0.06

- 0.04 0.01

-0.01 - 0.08 -0.01 - 0.03 0.03

- 0.00 0.00 0.00

- 0.03 - 0.04 0.03

-0.01 - 0.02 0.00 0.00 0.05 0.02

- 0.01 0.01 0.01 0.01 0.02

- 0.06 0.01 0.03 0.02

- 0.04

28 645.98 27 780.43 29 223.12

-0.01 0.01

- 0.00

36 182.82 35 115.56 28 756.30

- 0.04 0.04 0.04 28 735.41 0.01 28 714.29 0.05

- 0.03 -0.01

34 895.59 29 485.60 36 633.62 26951.00 37 310.86 28 127.79 38 102.44 27 599.09 29 126.70 33 013.62 28 970.20 35 482.23 31 716.34 34 772.75 29 741.99 33 070.13 29 741.08 36 745.69 33 028.53 36 129.78 38 295.73 33 754.63 37061.22

0.00 - 0.02 0.02

- 0.0 1 - 0.0 1 - 0.0 1 -0.01

0.0 1 - 0.00 0.02

-0.01 0.03

- 0.02 - 0.0 1 0.06 0.04

- 0.03 0.02

- 0.09 0.00

-0.01 - 0.05 0.01

34 874.06 29457.12 29 428.9 1

26 906.44

- 0.0 1

0.00 26 928.85 37 298.09 28 099.32

0.0 1 -0.01 - 0.03

27 573.87 29 107.42 32 993.55 28 949.72 35 452.34 31 692.17 34 736.20

0.04 0.02 0.02 0.0 1 0.00 0.01

-0.01

29 087.30 0.07

35 422.50 31 667.48 34 700.13

0.03 0.00

-0.01

33 047.32 0.00

36 723.77 33 005.41 36 103.90 38 264.48 33 729.68 37 035.24

0.03 - 0.02

0.0 1 - 0.00 - 0.08 0.02

36077.18 38 232.89 33 703.40 37 007.99 33 703.40 36 998.05

- 0.07 -0.01 - 0.07 - 0.02 0.09 0.01 37 025.38 0.01

62 952.14 - 0.02

37 767.82 37 767.82

0.01 0.04

37 739.85 37 739.85

-0.01 0.02

37 710.33 37 710.33

- 0.0 1 0.2

34 378.21 34 378.21

-0.01 - 0.03

33 502.65 0.0 1 33 485.50

33 476.61

0.0 1

-0.01

33 470.38

33 461.28

- 0.04

- 0.03 33 493.93 0.02

61 846.86 61 846.86 36 847.30

0.00 0.00 0.0 1 36 829.21 - 0.00 36 813.16

36 8 10.98 35 723.46 35 725.99 36 387.42 32 459.1 1 36 376.54 32 296.87 35 867.46

31 314.41

0.05

0.04 0.0 1 0.00 0.01

- 0.04 - 0.09 0.05 0.05

0.00

36 845.19 35 630.20 35 632.83 36 425.3 1 32499.16 36414.84 32 343.38 35 909.92

0.03 -0.01 - 0.02

0.01 - 0.01 - 0.00 0.01

- 0.05

36 827.03 35 678.18 35 680.64 36 405.24 32 477.98 36 394.51 32 319.04 35 887.53

0.03

0.06 0.0 1 0.00 0.0 1

- 0.10 - 0.04

- 0.6

31 376.75 38 064.82 38 069.89

0.02 0.00 0.0 1

31 344.12 0.03

35 301.13 35 136.72

- 0.03 0.0 1

35 276.38 35 108.27

0.04 0.00

35 254.3 1 35 082.56

- 0.00 - 0.00

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Table 1 Continued

351 35, A B

4 l , 3 ' 3 0 . 3

4 2 . 2 ' 3 1, 2

4 2 . 3 ' 3 1 . 3

5 1 . 4 ' 4 0. 4

5 2 . 3 ' 4 1. 3

5 2 . 3 ' 4 2. 2

5 3 . 2 ' 4 2 . 2

5 3 . 2 ' 4 3 . 1

5 3 . 3 ' 4 3 . 2

5 4 . 1 ' 4 3 . 1

5 4 . 1 ' 4 4. 0

5 4 . 2 4 - 4 4 , l

6 1 , 5 + - 5 1 . 4

6 2 . 4 ' 5 2. 3

6 2 . 5 ' 5 2. 4

6 3 . 3 ' 5 3 . 2

6 3 . 4 ' 5 3 . 3

6 4 . 2 ' 5 4, 1

0 . 7 ' 0 . 6

1 . 6 ' 1 . 5

7 1 . 7 ' 1. 6

7 2 . 5 ' 6 2 . 4

7 3 . 5 + 6 3 . 4

7 4 . 4 ' 6 4 . 3

2 . 6 ' 2 . 5

0.8' 0 . 7

1 . 7 ' 1 . 6

1 . 6 ' 1 . 7

2.7' 2 . 6

0.9' 0 . 8

1 . 9 ' 1 . 8

l4 3 . 1 1 ' l4 5 . 1 0

l4 4 . 1 1 ' l4 4.10

l 5 0 . 1 5 l4 0 . 1 4

l 5 1 . 1 5 ' l4 1 . 1 4

l5 3 . 1 2 ' l 5 5 . 1 1

l 5 4 . 1 2 ' l 5 4 . 1 1

l 5 12, 3 ' l5 13, 3

l5 12. 4 ' l 5 13. 2

l 6 4 . 1 2 ' l6 6 . 1 1

l 6 5 . 1 1 ' l6 7 . 1 0

l6 5 . 1 2 ' l6 5 . 1 1

l6 6 . 1 1 ' l6 6. 1 0

l7 5 . 1 2 l7 7 . 1 1

l 7 7 . 1 1 + l7 7 . 1 0

l7 1 3 . 4 l7 1 4 , 4

l7 1 3 , 5 ' l7 14. 3

l8 6 . 1 2 ' l8 8 . 1 1

l8 7 . 1 2 ' l8 7 , 1 1

28 643.58 27 778.65 29 213.40 36 176.28 35 116.98 28 754.48 34 887.58 29 498.00 26 954.01 37 282.66 28 141.88 27 607.43 29 119.83 33 003.41 28 965.87 35 488.40 31 716.00 34 793.13 29 737.97 33 063.68 29 737.04 36 733.09 33 023.17 36 125.78 38 299.71 33 750.15 37 054.74 33 750.15 37 045.35 37 762.89 37 762.89 33 495.42 33 487.02 61 839.20 61 839.20 36 837.83 36 835.81 35491.80 35 494.59 36 420.24 32 50 1.07 36 410.19 32 35 1.79 35 909.93 3 1 402.23 37912.91 37 918.23 35 306.53 35 149.56

- 0.02 0.01

- 0.02 0.07

- 0.02 0.06

- 0.03 -0.01 - 0.03 -0.01

0.0 1 0.0 1

- 0.02 - 0.02 - 0.02

0.04 0.0 1

- 0.00 0.08

- 0.05 - 0.06 - 0.05

0.03 0.00

- 0.02 - 0.06 - 0.05

0.09 0.04 0.00 0.03

- 0.04 0.03

- 0.00 - 0.00 - 0.02

0.01 0.01

- 0.03 - 0.04 - 0.0 1

0.03 - 0.00

0.02 0.04 0.01 0.01

- 0.03 0.0 1

28 731.44 0.03 28 774.41 0.06

29 512.16 26 969.92

-0.01 -0.01

29 51 1.76 26 975.50

0.05 0.03

28 153.9 1 27 62 1.56 29 140.44 33 029.51 28985.17

0.02 0.02

- 0.05 0.02

- 0.04

27 589.42 29 093.87 32 973.51 28941.10 35 463.57 31 690.95 34 774.59

0.06 -0.01 - 0.04

0.05 0.04

- 0.02 - 0.02

29 154.15 33041.16 28 997.97

- 0.03 0.03

- 0.04

31 736.25 34 806.98

- 0.02 -0.01

31 745.05 34 804.72

- 0.03 - 0.02

33 034.70 0.02 33 086.22 0.7 33 102.32 0.07

36 699.17 32 994.43 36 095.60 38271.40

- 0.02 - 0.04 - 0.09 - 0.08

36 762.06 33 045.06 36 150.20 38 322.67 33 770.42 37 079.33 33 770.42 37 069.68 37 785.54 37 785.45 33 532.36 33 523.81

- 0.0 1 - 0.06

0.0 1 0.00

- 0.08 0.04 0.7

-0.01 - 0.02

0.0 1 - 0.02

0.02

36 777.47 33 060.85 36 162.68 38 329.95 33 791.22 37 097.97 33 791.22 37 088.28 37 808.98 37 808.98 33 506.12 33 497.41

0.00 - 0.05

0.01 - 0.08 - 0.07 -0.01

0.08 - 0.0 1 - 0.00

0.02 - 0.02

0.03

37 022.43 33 720.68 37 01 3.04 37 729.92 37 729.92 33 470.57 33 462.40

0.04 0.06 0.02

-0.01 0.0 1 0.0 1 0.0 1

36 809.93 36 808.00 35 412.19 35 414.90

- 0.0 1 - 0.03 - 0.04

0.04

36 879.15 36 877.07 35 590.83 35 593.37 36 459.1 1 32 532.79 36 448.89 32 380.42 35 945.99 31 422.19

- 0.0 1 0.03

- 0.08 0.08

- 0.00 0.03

- 0.00 0.03

- 0.02 -0.01

36 850.9 1 36 848.8 1 35 61 5.85

- 0.03 -0.01

0.00

36 429.42 32 503.67 36419.00 32 348.86 35 914.68 31 384.47

0.02 0.06 0.03

- 0.02 - 0.04 - 0.02

32 480.72 36 384.59 32 334.10 35 886.64

-0.01 0.02 0.00

- 0.03

35 285.82 35 131.81

- 0.05 0.07

35 339.48 35 179.01

- 0.00 0.00 35 143.16 0.00

Table 2 inertia, P, , for the ground and excited vibrational states of 5,6-dihydro-2H-thiopyran

Rotational constants (in MHz), quartic centrifugal distortion constants (in kHz), vibrational wavenumbers and planar moments of

4213.808 (2)" 3236.812 (1) 2008.2265 (7)

0.831 (6)

0.95 (1) 0.066 (2)

- 1.59 (1)

-0.69 (2) 70 0.03

12.207 -

4213.136 (2) 3233.956 (1) 2006.7896 (7)

0.833 (6)

0.92 (2) 0.070 (2)

- 1.57 (2)

-0.65 (3) 52 0.03

134 (12) 12.196

4212.352 (4) 3231.776 (1) 2005.266 (3)

0.82 (2)

0.91 (3) 0.068 ( 5 )

- 1.57 (4)

-0.65 (6) 37 0.03

12.180 -

421 1.432 (3) 3228.364 (3) 2003.653 (4)

0.83 (2)

0.92 (2) 0.066

- 1.59 (1)

- 0.69 30 0.05

12.158 -

4208.745 ( 5 ) 3234.776 (1) 2006.564 (1)

0.83 (1)

0.90 (2) 0.069 (3)

- 1.55 (3)

-0.63 (4) 49 0.04

219 (18) 12.225

4203.862 (3) 3232.656 (3) 2004.817 (3)

0.85 (2)

0.91 (1) 0.066

- 1.56 (1)

-0.69

0.04

12.236

31

-

4214.00 (1) 3236.670 (3) 2007.703 (3)

0.83 (3)

0.92 (4) 0.070 (9)

- 1.58 (6)

-0.67 (8)

0.04 339 (24)

12.175

35

4215.135 (3) 3236.546 (3) 2009.071 (3)

0.83 (2)

0.93 (2) 0.066

- 1.59 (1 )

- 0.69

0.04 412 (18)

12.248

31

a Standard error in units of the least significant digit. Number of transitions in fit. Standard deviation of fit.

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Table 3 Zero-field line frequencies for the transitions used in the electric dipole moment determination, and the electric dipole moment components and total electric dipole moment for $6- dihydro-2H-thiopyran

transition v,/MHz

electric dipole moments" P a

pb P c Pr

16990.29 15735.07 17631.10

1.35 (1)

0.74 (2) 1.54 (2)

<0.1

" In D.

accurately predicted. The transitions used in the electric dipole moment determination are listed in Table 3 together with the derived electric dipole moment components and the total dipole moment. The total electric dipole moment of 1.54 (2) D may be compared with that of 1.50 (1) D in dimethyl sulfide" and that of 1.78 (1) D in thiane."

Molecular Conformation and Discussion If 5,6-dihydro-2H-thiopyran has a planar heavy-atom equi- librium conformation the ground vibrational planar moment of inertia,'

h

apart from a small vibrational contribution, would be due to the out-of-plane methylene hydrogen atoms and would have a value of 4.9 x lo4 u pm2.t The observed value of 12.2 x lo4 u pm2 is considerably larger and leaves little doubt that 5,6-dihydro-2H-thiopyran has a non-planar heavy-atom skeleton. A model structure based on that of c y ~ l o h e x e n e ' ~ ~ ' ~ (with the C-S bond lengths and <CSC angle taken from saturated six-membered ring molecules con- taining a sulfur atom"*'5) was used as a starting point for the determination of the molecular conformation. Starting from the planar ring conformation, the model structure was distorted first along the twisting and then along the bending coordinate. These calculations indicated that only a twisted conformation was likely to be able to reproduce the observed rotational constants. In order to have more quantitative information about the molecular structure, least-squares fitting to the ground vibrational state rotational constants and planar moments of inertia using the method of predicate Observations' was adopted. In this method, estimates of some or all of the parameters to be determined, the predicate observations, are added to the experimental data. This ensures that, when the number of parameters to be deter- mined is larger than the number of experimental data, the problem of the singularity of the least-squares normal equa- tion matrix is removed.

The definitions of the twisting (z) and bending (p) coordi- nates given by Wells and Malloy17 have been used. The num- bering of the ring atoms is shown in Fig. l(a). A Cartesian coordinate system is chosen with the origin at the mid-point of the C(2)-C(5) diagonal and with the x axis along this diagonal. The y axis is chosen to lie in the plane of the C(2)C(3)C(4)C(5) atoms. Because of the lack of symmetry of the non-planar ring conformers, it is helpful to visualize these using the projections of the S-C(6) bond in the xz and y z

t 1 u z 1.66054 x kg.

2

5

I = I s

I = , ;:!GS , 0

0 . /0 ' -3

t Z s

Y

Z +

P Y

( e 1 Fig. 1 (a) Numbering of the ring atoms in 5,6-dihydro-2H-thiopy- ran. Projection of the S-C(6) bond in (b) the xz plane for a pure twist conformation, (c) the y z plane for a pure bent conformation and (d) , (e) the xz and yz planes for a mixed conformation.

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planes and the coordinates of the point (0') of intersection of this bond with the y z plane. In a pure twisted conformation [Fig. l(b)] the projection of the S-C(6) bond passes through the origin and the coordinates of a point on the bond are related by

(2) x tan(z) - z = 0

In a pure bent conformation [Fig. l(c)] the coordinates of a point on the S-C(6) bond are related by

y tan(p) - z = 0 (3) A mixed conformation may be achieved, starting from the planar ring, first by twisting the S-C(6) bond by t about the y axis and then rotating 0-0' by p about the x axis. For this type of conformation, the coordinates of a point on the S-C(6) bond are related by

(4) A series of preliminary computations using the experimental data and structure of cy~lohexene'~ was made in order to choose the weights to be given to the rotational constant and planar moments of inertia. Satisfactory results were obtained with a weight of 0.1 for the rotational constants and 1.0 for the planar moments of inertia. Each predicate observation was given a weight equal to the reciprocal of its estimated uncertainty. For 5,6-dihydro-2H-thiopyran the predicate observations and their uncertainties were obtained from a survey of the structures of cyclohexene and related molecules and from the structures of ring molecules containing sulfur atoms.

Table 4 gives the results of fits for 5,6-dihydro-2H-thiopy- ran, In all of these fits the following restrictions were placed on the molecular structure: (i) equality of the C(2)-C(3) and C(4)-C(5) bond lengths; (ii) equality of the two H-C(sp2) bond lengths; (iii) equality of all the H-C(sp3) bond lengths; (iv) local C,, symmetry at all C atoms; (v) equality of all <HCH angles. In fit (a) the further restriction of a pure twisted conformation was also imposed. In fits (b) and (c) this restriction was removed. In fit (b), the dihedral angles of the C(2)-S (&) and the C(5)-C(6) (&(6J bonds with respect to the plane of the C(2)C(3)C(4)C(5) atoms were fitted rather than determining j3 and z directly. In fit (c), z and j3 were determined directly. The calculations give very similar values

x tan(t) + y sin(p) - z cos(p) = 0

of z, and this appears to be the best determined feature of the structure. The experimental data do not depend significantly on p. This is shown by fits of type (c) where the partial derivatives of the experimental data with respect to j3 are small and the final value of p is close to its predicate value even for appreciable values of /3. The estimated uncertainty in t (three standard errors) is ca. 1.5".

The reasonableness of the structure may be judged by examining the ring parameters that were not fitted but may be calculated from the fitted parameters. These are also given in Table 4 together with their errors, estimated from the least-squares variance-covariance matrix. The S-C(6) bond length is equal within its calculated error to that of the S-C(2) bond. These bond lengths are expected to be very similar because they are both of the type S-CH,. The <C,SC, angle is similar to that in thiane" and other four-, and five- and six-membered ring molecules containing a sulfur atom. The <SC(6)C(5) angle is close to the tetrahedral angle for an sp3 carbon. The requirement of near equality of the two S-C bond lengths makes it unlikely that the absol- ute value of j3 is greater than 10". For negative predicate values of 8, the S-C(6)- becomes longer than S-C(2), while for positive values it becomes shorter. Also, for appre- ciable predicate values of /3 the ethenic ring angles become quite different, as do the ring angles at C(2) and C(5).

Table 5 gives a comparison of the twist angles in 5,6- dihydro-2H-thiopyran and its oxygen analogue (which is called 3,6-dihydro-2H-thiopyran in ref. 17 and 19), as deter- mined from microwave spectroscopy,' ' molecular mechanics c ~ m p u t a t i o n s ' ~ ~ ' ~ and the minima of the IR two-dimensional (2D) vibrational potential-energy The trend of the results is the same for both molecules with the microwave and molecular mechanics values of z being very similar and somewhat smaller than the IR value. In spite of the large differences between C-0 and C-S bond lengths and < COC and < CSC bond angles, the equilibrium twist angles of the two molecules are not very different. The preference for the twisted conformation in cyclohexene and similar mol- ecules can be attributed to the need to accommodate the C-C bond, and the much longer single bond across the ring from it, without increasing the ring-angle strain energy unnecessarily. This can be achieved more efficiently in the twisted conformation than in a bent one.

Fig. 2 gives stick diagrams showing the vibrational depen-

Table 4 Predicate observations and final molecular structures for 5,6-dihydro-2H-thiopyran (bond lengths in pm and angles in degrees)

(4 (4 (4

predicate final predicate final predicate final

181.5 (2.0)" 150.5 (1.0) 133.5 (1.0) 152.5 (1.0) 108.5 (1.0) 109.5 (1.0) 113.0 (5.0) 123.0 (5.0) 123.0 (5.0) 113.0 (5.0) 109.5 (4.0) 30.0 (5.0) 0" 9.8'

16.3' 138.0' 103.6' 118.3'

181.5 (1.5)b 150.5 ( 1 . 1 ) 133.7 (1 .1) 152.5 ( 1 . 1 ) 108.5 (1 .1) 109.6 (1.1) 114.8 (1.1) 125.5 (1.4) 124.8 (1.2) 1 1 3.9 (1.6) 109.5 (2.2) 32.5 (0.4) 0'

15.3 (0.5)' 22.5

182.1 (2.6)' 97.4 (0.6)'

109.2 (1.2)d

181.9 (1.4) 151.0 (1.0) 133.8 (1.0) 152.6 ( 1 .O) 108.5 (1.0) 109.6 (1.0) 114.5 (1.0) 125.4 (1.3) 125.0 (1.0) 113.8 (1.4) 109.3 (2.1)

36.4' 32.6 (0.4)'

15.0 (5.0) 16.1 (1.3) 20.0 (5.0) 21.1 (1.3)

154.9' 180.9 (2.3)' 98.5' 97.3 (0.6)'

1 10.6' 109.9 (1.1)'

1.9' 1.2 (1.2)'

181.5 (1.5) 150.8 ( 1 . 1 ) 133.7 ( 1 . 1 ) 152.5 (1 .1) 108.5 (1.1) 109.6 (1.1) 114.8 (1 .1) 125.5 (1.4) 124.9 (1.2) 113.9 (1.6) 109.5 (2.3)

30.0 (5.0) 32.5 (0.4)

9.8' 15.5 (1.8)'

138.V 182.0 (2.6)' 103.6' 97.4 (0.7)' 1 18.3' 109.3 (1.4)d

0.0 (5.0) 0.2 (1.8)

16.3' 22.2 (2.2)d

~~ ~~~

Unless indicated otherwise the predicate observations for fits (b) and (c) are the same as for fit (a). a Estimated uncertainty. ' Standard error. ' Parameter calculated from fitted parameters. Estimated uncertainty calculated from the variance-covariance matrix. " Constrained value.

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Table 5 Comparison of the twist angle, z, as determined by microwave spectroscopy, molecular mechanics calculations and from the minima of the IR potential-energy surfaces for 5,6-dihydro-2H-thiopyran and its oxygen analogue, 3,6-dihydro-2H-pyran

microwave spectroscopy molecular mechanics IR spectroscopy

5,6-dihydro-2H-thiopyran 32.5 (1.5)11 30Sb 37.tIb 33.6' 38.2d 3,6-dih ydro-2H-pyran 31.5 (3.0)E

This work. Ref. 2. Ref. 17. ' Ref. 19.

dence of the rotational constants. From these diagrams, on the basis of both the regular changes in the rotational con- stants and the relative intensities of the vibrational states (the relative heights of the sticks), it is possible to assign the pro- gressions 36,, 362, 36, and %,, 35, in the two lowest- wavenumber vibrations. The fundamental wavenumbers of v36 and v35 given in Table 2 are in very good agreement with the values of 139.0 and 235.6 cm-', respectively, from the IR spectrum.

0.8 1

B

3225 3230 3235 BIMHz

1.0 I

I (c'

3240

0

B

1 0

C / M H t Fig. 2 Stick diagrams showing the variation of the rotational con- stants with vibrational state: (a) A, (b) B, (c) C. The stick heights are equal to the relative intensities of the vibrational states.

The assignment of the two highest-wavenumber states labelled A and B in Table 2 is not so obvious. The wavenum- ber of state A is consistent with it belonging to either the first excited state of the third lowest vibration, v 3 4 , or to the com- bination state, 35,36,. The fundamental of v34 has been observed at ca. 360 cm-' in the Raman spectrum of liquid 5,6-dihydro-2H-thiopyran2' while the combination state 35'36, should have a wavenumber of ca. 353 cm-'. The rota- tional constants of A are somewhat different from those expected for 35,36, on the basis of the additivity of the differ- ences in the rotational constants of 35, and 36, and those of the ground vibrational state. For this reason state A may be tentatively assigned to 34,. The wavenumber of state B is in reasonable agreement with bands at ca. 383 and ca. 453 cm- ' in the liquid-phase Raman spectrum." It is somewhat lower than the ca. 500 cm-' expected for the combination states 34,36, or 35,36,. It therefore appears that B may be the first excited state of the fourth or fifth lowest vibration ( v 3 3 or

The assignment of the ring-bending and ring-twisting vibrations for 5,6-dihydr0-2H-thiopyran'~~' was made on the basis that v36 and v35 occur ca. 40 cm-' below the ring- bending and -twisting vibrations of cyclohexene. An attempt to confirm the assignment of these vibrations, via an exami- nation of the changes in the rotational constants and planar moments of inertia upon excitation of v36 and v , ~ was largely inconclusive. The simple models for the twisting and bending of the ring, with the planar ring as a reference conformation, predict that the three rotational constants should decrease and the planar moments of inertia increase as the ring is dis- torted along either z or p. If the equilibrium conformation is taken as a reference, the rotational constants are still expected to decrease and the planar moments of inertia increase as z is increased. As p is changed about this refer- ence, A and P b are expected to behave differently from the other two rotational constants and planar moments of inertia. Experimental excitation of v35 leads to a decrease in the three rotational constants and an increase in the three planar moments of inertia. This is the behaviour expected for twisting the ring about either reference. Excitation of v36

leads to a decrease in the three rotational constants, although the change in A is smaller than those in B and C. The change in P , is negative while those in Pa and P , are positive. This is not the behaviour expected for bending the ring about either of the reference conformations. In four- and five-membered ring molecules the planar moment of inertia, P , , usually increases upon excitation of the ring-puckering vibrations because the vibrationally averaged non-planarity of the ring increases. A similar decrease in P , has been found upon exci- tation of the lowest-wavenumber vibration in 4- methylenecyclohexene.2

When, as in the present case, the barrier separating the equivalent equilibrium conformers is very high, the lowest vibrational states consist of almost degenerate pairs of vibra- tional levels. This is often referred to as inversion doubling. The difference in energy between the members of a pair of nearly degenerate level depends on the potential-energy surface, the reduced mass for motion on this surface and the vibrational wavefunctions. The pa and p b components of the

v32)*

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electric dipole moment are symmetric while the p c com- ponent is antisymmetric with respect to inversion of the ring (z, B + -7, -p). The p a and & components of the electric dipole moment give rise to pure rotational semirigid rotor transitions, but the ,uc component gives transitions between the rotational levels of the two nearly degenerate vibrational levels. The pc transitions occur in pairs at frequencies of v,,, f vinv, where v,,, is the semirigid rotor frequency of the pure

rotational pc transition and vinv is the frequency correspond- ing to the energy difference between the almost degenerate vibrational levels. In the present work, some pc transitions have been observed for the ground and all of the excited vibrational states and none of the observed transitions showed any splitting. The inversion splitting in the states 36, and 35, must be smaller than CQ. 0.1 MHz. This implies a very high barrier to ring inversion, in agreement with the conclusion from the IR spectrum.2 Since the pc electric dipole moment connects different inversion states, the quantity listed as pc in Table 3 is really the transition moment ( O + I pc lo-) z (0- I pc 10'). The small value of Ainv jus- tifies its neglect in the calculation of the perturbation sums involving pc in the determination of the Stark coefficients.

In conclusion, it can be stated that the microwave spec- trum of 5,6-dihydro-2H-thiopyran confirms the twisted or half-chair equilibrium conformation of the molecule deduced from its vibrational spectrum and molecular mechanics com- putations.2 The microwave spectra of the excited vibrational states also confirm the fundamental wavenumbers of the two lowest vibrations. As found previously2' the changes in the rotational constants and planar moments of inertia upon excitation of the ring-puckering vibrations in some six- membered ring molecules appear to be smaller and more dif- ficult to interpret than those for five-membered ring molecules.

We thank the following for financial support: The comision Asesora de Investigacion Cientifica y Tecnica (CAICYT, grant PB90-345) (L.A.L. and J. L. A.). The University of Val- ladolid, the MURST 60% funds and the CNR (D.G.L.). The European Union 'Human Capital and Mobility' Program (Contract CHRX CT 93-0157) (L.A.L, J.L.A. and D.G.L.).

The National Science Foundation and the Robert A. Welch Foundation (M.M.J.T., J.R.V. and J.L.).

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Paper 4/02 163K ; Received 12th April, 1994

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