Microwave spectrum and ring puckering motion in thiazolidine

JOURNAL OF MOLECULAR

SPECTROSCOPY

137,354-36 l (1989)

Microwave Spectrum and Ring Puckering Motion in Thiazolidine WALTHER CAMINATI AND SALVATORE DI BERNARDO Centro di studio di Spettroscopia a Microonde, Dipartimento di Chimica “G. Ciamician, ” Universitd di Bologna, Via Selmi 2, I-40126 Bologna, Italy; Istituto di Spettroscopia Molecolare de1 C.N.R., Via de’ Castagnoli 1, I-40126 Bologna, Italy

The microwavespectrumof a ringtwistedconformationof thiazolidinewith the amino hydrogen in the axial position has been assigned. Information on the ring-puckering motion has been obtained from an analysis of the rotational spectra of some vibrational satellites. The electric dipole moment and the quadrupole coupling constants have been determined. 0 1989 Academic Press, Inc.

INTRODUCTION

Thiazolidine (THZ) is a molecule important from a biochemical point of view. For example, it has been claimed that it acts as a determinant of cat neural taste response to meats (I ) and that as a chemical precursor of carcinogenic N-nitrosothiazolidine it has been detected in fresh sardines (2). Nevertheless, no structural information is available in the literature, and the only spectroscopic investigation is a Raman study (3). No information is available for the lower-energy motion due to the ring puckering. Information on this motion, obtained by spectroscopic investigations, has been reported for several saturated five-membered rings (see, for example, the review by Legon (4)). Among the saturated five-membered rings we were especially interested in pyrrolidine and tetrahydrothiophene, the two molecules most similar to THZ, as they could be good references in starting the analysis of the microwave spectrum. For both of them the rotational spectrum has been investigated (5, 6). Furthermore, data based on electron diffraction and ab initio investigations are available for pyrrolidine ( 7). While pyrrolidine adopts an envelope configuration with the axial amino hydrogen (5, 7), tetrahydrothiophene has a twist shaped ring (6). Figure 1 displays the plausible conformations of THZ and the numbering of the atoms that will be used through the text. EXPERIMENTAL

DETAILS

THZ was purchased from Aldrich Chemical Inc. and was used without further purifications. The N-D monodeuterated species was obtained by direct H/D exchange in a cell saturated with D20. The microwave spectra have been obtained with a conventional Stark modulated spectrometer driven by a computer (8) in the frequency range 18-40 GHz. Relative intensity measurements have been done by using the Esbitt-Wilson method (9). 0022-2852189

Copyright 0

$3.00

1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

354

355

THIAZOLIDINE

AX.

EQ.

TWIST

BENT

FIG. 1. A sketch of the more plausible conformations of THZ and atom numbering.

RESULTS AND DISCUSSIONS

(a) Assignment of the Rotational Spectra As no experimental or ab initio structural data were available in the literature, the first trial calculations of the rotational constants were based on the geometries of pyrrolidine ( 7) and tetrahydrothiophene (6). The main problem was that while pyrrolidine adopts an envelope configuration, tetrahydrothiophene is in a twist form. Furthermore, the amino hydrogen can be axial or equatorial. We calculated the approximate spectra for all four possible configurations arising from a twisted or envelope configuration and an axial or equatorial position of the amino hydrogen, that is, the four conformations of Fig. 1. The corresponding calculated trial spectra were similar to each other, with the K asymmetry parameter in the range from -0.09 to -0.15. The radiofrequency-microwave double resonance technique (10) was helpful in assigning the first transitions. Typically a JIJ * Jo,,, PC-type transition was used to pump a Jx,J f (J - 1),,>_i punor &-type transition (x and y were 0 or 1) with J in the range from 4 to 6 and the corresponding radiofrequency in the range from 6 to 150 MHz. Then several transitions have been measured with the normal Stark modulation technique. In the same way, the rotational spectra of some ring-puckering excited states and of the ground states of the N-D and S(34) isotopic species have been assigned and measured. All measured transitions are listed in Table I. Some of these are center frequencies corrected for hyperfine structure due to 14Nquadrupole coupling effects. Table II reports the spectroscopic constants obtained by treating the experimental frequencies with the quartic Watson Hamiltonian ( 11) .

356

CAMINATI AND DI BERNARD0 TABLE I HyperIine Corrected Center Frequencies of All Measured Transitions (MHz)

(b) 14NQuadrupole Coupling Efects The 14N nuclear quadrupole hyperfine structure was analyzed for some transitions which show splittings large enough to be observed with our spectrometer. The measured splittings are reported in Table III along with the obtained quadrupole coupling con-

THIAZOLIDINE TABLE II Spectroscopic Constants of Thiazolidine

I

F2-F,

-0.58

--

2C2.0)

F3-F2

0.88

0.893

3C2.2)

--

2(2,1)

Fz-FI

-0.57

0.558

5C5.1)

--

5t4.21

F2-F,

b

0.62

0.616

5C5.0)

--

5C4.1)

F2-F,

b

0.65

0.637

7C5.3)

--

7C4.4)

F2-F,

b

0.27

0.301

7C5.21

--

7C4.31

F2-F,

b

0.36

0.375

8C6.2)

--

8C5.3)

F2-F,

b

0.38

0.351

8(5,4)

--

8C4.5)

F2-F,

b

0.21

0.207

2a8=-1.73(4)

a

a

3(2,11

For J>2 e"e~y strong

AF=hl

observed

rotational qusdrupole

only

Ybb=O.78(8)

this kind

transitions component

of quadrupole

end we use F,, F2 end F3 to with

the highest.

label

intermediate

has

lines.

and

three

ue

have

components, the

component

lowest

Values

of F. respectively. b

For

these

within

our

component

transitions experimental line.

Fl

is

always

resolution,

overlapped, to

the

F3

358

CAMINATI AND DI BERNARD0

stants. They agree with the values obtained by transferring the quadrupole coupling constants of ammonia to the structure of the axial conformer. (c) Dipole Moment Table IV reports the displacements, due to an applied electric field, and the Stark coefficients (12) of the Stark lobes of some chosen transitions. All lobes showed a second-order Stark effect, and it was possible to obtain the dipole moment components listed at the bottom of Table IV. The cell was calibrated with the 2 * 1 transition of OCS (/L = 0.71521 D (13)).

(d) Conformation and Structure The values of the rotational constants and the substitution coordinates ( 24) of the amino hydrogen and of the S atom (see Table V) correspond to the twist-axial conformation. No strong lines were left unassigned in the spectrum, so that we believe that other conformations, which should have dipole moments as large as that of the observed species, are not stable or lie at a much higher energy. It seems that, as a rule, TABLE IV Stark Coefficients and Dipole Moment of THZ

J'(Kg,

Kc)'--J"(K,.K,)

&V/E2

IXI

I

31,2<-21.1

I

3~,3'-2~.2

I

32.1<-2.0

1.258111)

D

&,

=

0.674(38)

D

#,

=

0.462(21)

D

v-2

Crn2)

1

-10.28

-10.45

2

-40.08

-40.14

0

-5.14

-5.07

1

-9.33

-9.38

9.37

10.02

0 1

-279.4

12

,'fa =

(HZ

-1233.

Pttot

=

1.500(34)

-282.3

-1159.

D

359

THIAZOLIDINE TABLE V Plausible Structure of Thiazolidine and rs Located Positions of the Amino Hydrogen and S Atom (A and “) I 1 Full

geometry

Bond

(See

text). Valence

lengths

angles

Dihedral

angles

Sl C2

1 .a29

SlC3

1 .829

c3slc2

N4C2

1.471

N4C2s1

109.53

N4C2-SlC3

13.21

C5C3

1.544

C5C3Sl

104.51

c5c3-slc2

13.21

CH

1.09

HCH

106.00

NH

1.02

HNB

21

Amino

hydrogen

and

S

atom

92.01

a)

rs

120.65

coordinates. CalC.

exptl.

twist-ax. la HN

bent

I

1.4103(3)

1.411

2.159

1.381

2.205

0.624017)

0.624

1.058

0.673

1.005

Ic I

1.2578(3)

1.255

0.179

1.274

0.155

lb

I I

Ic I

a)

bent-ax.

Ibl

la S

twist-eq.

B represents

1.2505(2)

1.249

1.254

1.244

1.250

0.059(41

0.039

0.089

0.007

0.133

0.013l15)

0.000

0.011

0.047

0.038

the

direction

of

the

bisector

of

the

C3N4C5

eq.

angle.

in saturated five-membered rings containing an NH group, the amino hydrogen prefers the axial position (5). Table V reports a partial plausible r. structure that has been obtained by fitting the nine available ground state rotational constants. The bond lengths have been fixed to the values from Refs. (6, 7) and the HCH angles have been fixed to 106’ (from Ref. 6). Furthermore, the methylenic and amino hydrogens have been kept in the plane perpendicular to the plane containing the heavy atom to which they were attached and the two adjacent heavy atoms, and containing the bisector of the angle formed by these three heavy atoms. Finally, we imposed that the two dihedral angles defining the positions of N4 and C5 were equal to each other. This last restriction corresponds to the choice of the twist form. Actually, it would have been possible to reach a fitting of the rotational constants also by adopting a bent configuration of the ring, but obtaining exaggerately distorted valence angles of the ring and a poor agreement between experimental and calculated r, coordinates of the amino hydrogen (see Table V). The reported plausible structure accounts satisfactorily also for the r, coordinates

360

CAMINATI AND DI BERNARD0

FIG. 2. Schematic representation of a transformation between two equivalent minima.

of the S1 and H6 atoms. We cannot, of course, exclude a distortion from the “exact” twist form toward the bent form. (e) Ring Puckering The observed conformation must exist in two equivalent forms as shown in Fig. 2. This means that two equivalent minima are expected in the potential energy surface and that they can interconvert through an appropriate tranformation. One path could be represented by a twisting involving N4 and C5 combined with the amino hydrogen inversion. A second way could follow a pseudorotation-like model ( 7, IS). In the first case the ring should reach an almost planar configuration of the heavy atoms and we would expect the corresponding vibrationally excited states to be “more planar” than the ground state, while in the case of the pseudorotation this trend to “planarity” is not required. The second moments of inertia (M, = 1 (-I, + Zb+ I,), etc.) give the mass extension of a molecule along the three axes. They are reported for THZ in Table VI for the ground state, along with the shifts of these quantities upon ringpuckering excitation. The increase of M, and the decrease of it4& upon excitation suggest that the second path is the one followed. If we adopt the pseudorotation model, then the vibrational energies of the excited states (see Table II) suggest a high barrier to pseudorotation ( 16). CONCLUSIONS

The data obtained from the microwave spectrum of THZ are interpreted in terms of a twisted conformation of the ring, with axial direction of the amino hydrogen TABLE VI Second Moments of Inertia and Their Shifts upon Ring-Puckering Excitation (u A*) VP=1

G.S.

*)

105.3164

-0.0514

“bb

71.0967

-0.0991

ncc

8.8478

Hea

a)

For state

the *re

excited given.

states

VP=2

-0.1053

shifts

-0.0745

-0.1869

0.1053

the

v,=3

-0.1421

0.2132

with

respect

0.2070

to

the

ground

THIAZOLIDINE

361

(conformation I of Fig. 1). In making a comparison with the two similar molecules tetrahydrothiophene and pyrrolidine we note that THZ takes from the first molecule the ring conformation, and from the second molecule the axial position of the amino hydrogen. The shifts of the second moments of inertia, with respect to the ground state, upon excitation of the ring puckering, suggest that the ring-puckering motion corresponds to a pseudorotation. The values of the dipole-moment components and of the 14N quadrupole coupling constants are in agreement with the proposed conformation. ACKNOWLEDGMENTS We thank Dr. R. Danieli and Mr. T. Bonfiglioli for help with the electronic equipment. Walther Caminati thanks Dr. Danieli also for proposing him as Incaricato di ricerca at the Istituto di Spettroscopia Molecolare de1 C.N.R. RECEIVED:

May 10, 1989 REFERENCES

1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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