Pure rotational spectrum and dipole moment of norbornadiene determined by microwave Fourier transform spectroscopy

JOURNAL

OF MOLECULAR

SPECTROSCOPY

(1988)

lM,249-257

Pure Rotational Spectrum and Dipole Moment of Norbornadiene Determined by Microwave Fourier Transform Spectroscopy B. VOGELSANGERANDA.BAUDER Laboratorium ftir Physikaiische Chemie, Eidgeniissische Technische Hochschule, CH-8092 Zurich, Switzerland Pure rotational transitions of norbomadiene and norbomadiene-2-‘%Z have been observed by pulsed microwave Fourier transform (MWFT) spectroscopy. The assignment of the spectra has been assisted by MWFT Stark experiments. Rotational constants and quartic centrifugal distortion constants have been fitted from the measured transition frequencies. The accidentally small dipole moment of p = p’c= 0.05866(9) D has been determined from Stark effect measurements. The rotational constants of both isotopic species have been used to calculate the substitution coordinates of the olefinic carbon. Q r988Academic RSS, Inc. 1. INTRODUCTION

During the last years norbomadiene (bicyclo[2.2.l]hepta-2,Sdiene) attracted many investigators because of its photosensitized valence isomerization to quadricyclane which seems to be of interest as a model for photochemical energy storage (I, 2). The structures of norbomadiene obtained by several gas-phase electron diffraction (ED) studies (3-6), partially oriented NMR spectra in a nematic liquid-crystal solvent (7, 8), and ab initio calculations (9) are not in good agreement with each other and depend strongly on the assumptions made in each case. The most detailed ED analysis of Yokozeki and Kuchitsu (3) led to two different sets of solutions mainly differing in the angle CIC7C4 of the methylene bridge which was found to be 92.2(4)” for structure I and 96.0(2)’ for structure II. The earlier ED studies (4-6) reported values between 92.0” and 100.1” for this angle, whereas the NMR studies (7, 8) tend to a value of about 92” and the ab initio calculations (9) to values between 91.7” and 92.0”. Also the question of whether the olefinic parts in norbomadiene are nonplanar has not yet been decided. Whereas in all ED studies these groups were assumed to be planar, a combination of proton coordinates obtained from NMR spectra with ED carbon coordinates resulted in an endo displacement of the olefinic hydrogen nuclei of about 3” (7). A model of norbomadiene with the numbering of the carbon nuclei is shown in Fig. 1. Rotational constants determined from microwave (MW) spectroscopy provide complementary information about the structure of norbomadiene. A combined analysis of MW and ED data was often found to improve the molecular structure as demonstrated for norbomene (10). No MW spectra of norbomadiene were reported up to now. The accidentally small permanent dipole moment makes their observation with conventional Stark spectrometers difficult. Pulsed microwave Fourier transform (MWFT) spectroscopy is now well established 249

0022-2852188 $3.00 Copyright 0 1988 by Afademic Press, Inc. All tights of reproduction in any form reserved.

250

VOGELSANGER

AND BAUDER

Flc. 1. Model of norbomadiene with numbering of the carbon nuclei and principal axes.

for measuring pure rotational spectra of molecules with very small electric dipole moments (21-15). However, Stark splittings were absent from spectra recorded with the standard MWFT. This fact rendered the assignment of rotational transitions more difficult. Only recently MWFT Stark experiments were used to determine dipole moments (14-17). In the present work the MWFT method was applied successfully to record the rotational spectrum of norbornadiene. MWFT Stark experiments were essential for the assignment of the transitions and for the determination of the dipole moment. 2. EXPERIMENTAL

DETAILS

A commercial sample of norbornadiene (Fluka, Switzerland, 97% purity) was purified by preparative gas chromatography. Thereby, the purity of the sample was raised to 99.8%. For all measurements, an Ekkers-Flygare-type MWFT spectrometer (18) with waveguide cells operating in the range 8- 18 GHz was used. Our design is similar to that described by Bestmann et al. (19). The 6-m-long waveguide cells contained the gaseous sample at a pressure of 6 mTorr (0.8 Pa). The cells were cooled to -70°C. Microwave pulses of 300 ns duration with a peak power of lo-40 W were used to polarize the sample gas. The molecular emission signal was amplified and downconverted to the 0- to 1O-MHz band and digitized at a rate of 20 MHz for 5 12 channels. The single pulse experiment was repeated at 20 kHz. Up to 10 million individual coherent emission signals during an integration time of up to 8 min were accumulated digitally for signal-to-noise ratio enhancement. After a Fourier transformation the conventional power spectrum was recovered over a range of 10 MHz with 256 points. For the Stark effect measurements, the simple waveguide cells of the MWFT spectrometer were replaced by a 2-m-long X-band Stark cell for the frequency range 8- 12 GHz and by a 2.5m-long P-band Stark cell for the range 12- 18 GHz. Both Stark cells carried a OS-mm-thick septum in grooves, electrically insulated with thin Teflon tapes. Details of the design were reported earlier (20). In order to reduce microwave reflections of the septum, 75-mm (X-band)- or 50-mm (P-band)-long dovetails were machined at both ends of the septum. Since the Stark effect is not used as a modulation in MWFT spectroscopy, dc voltages up to 3000 V were applied to the septum resulting in static fields of up to 6200 V cm-’ in the X-band cell and up to 8200 V cm-i in the

MWl=T SPECTRUM OF NORBORNADIENE

251

P-band cell. The fields in the Stark cells were calibrated with the J = 1 + 0 transition of OCS using the dipole moment of 0.7 15 19(3) D determined by Reinartz and Dymanus (21). 3. ASSIGNMENT AND ANALYSIS

Norbomadiene is an asymmetric top with asymmetry parameter K = -0.2203. The ED study (3) confirmed the CzVsymmetry of this molecule, which is consistent with results from Raman polarization (22) and infrared spectroscopy (23). This implies that the permanent electric dipole moment is parallel to the principal axis c. The MW spectrum was predicted from rotational constants calculated from the ED structures of Ref. (3). Low Jtransitions were expected in regions 400-MHz wide considering the uncertainties of the structural models. Due to the very rich MWFI spectrum of norbomadiene, a lot of transitions were found in those regions which were searched for each low J transition. The Stark effect was essential in selecting the proper transitions. The J = 1 + 0 as well as four J = 2 f 1 transitions were assigned first. The influence of the Stark field on the rotational 2(2, 0)-I( 1, 0) transition is shown in Fig. 2. Subsequently additional transitions with J values of up to 37 were assigned. Their transition frequencies are listed in Table I. From intensity measurements at different tempemtures these transitions were associated with the vibrational ground state. Finally 25 @ype transition frequencies were subjected to an iterative least-squares fit to determine the a

16152

16158

b M=rl

M=O

I’

FIG.

I’

I

*I

16152

16158

MHz

MHZ

2. Stark effect of the rotational 2(2,0)-l( 1,O)transition of norbomadiene. Six-megahertz sections out of the IO-MHz ranges of the MWFf power spectra, recorded with a sample pressure of 6 mTorr (0.8 Pa), temperature of -7O”C, microwave carrier frequency of 16 155 MHz, pulse power of 10 W, pulse width of 300 ns, and an integration time of 8 min (10’ pulses). (a) MWFT spectrum recorded without Stark field. (b) A Stark field of 6850 V/cm was applied.

252

VOGELSANGER

AND BAUDER

rotational constants and the quartic centrifugal distortion constants. The latter are defined according to Watson’s asymmetric reduction in a prolate Z’ representation (24). The differences between measured and calculated transition frequencies are inTABLE I Observed Pure Rotational Transitions (MHz) of Norbomadiene

J’(K;.K;)

l(

1.

-

a

J”(K1.K;)

0)

-

O(

0.

”

‘ohs

0)

b

obs -“CalC

7883.926

-0.002 -0.001

2(

2.

1)

-

1(

1.

1) b

16431.171

2(

2.

0)

-

1(

1.

0)

b

16155.026

0.001

2(

1.

1)

-

l(

0.

1)

b

15104.519

0.001

2(

0.

2)

-

l(

1.

0)

b

12358.558

0.030

3(

1.

3)

-

2(

2.

1)

16412.171

O.CCO

12643.082

-0.001

9(

8.

2)

-

9(

7.

2)

12569.210

-0.001

lO(

8.

3)

-

lO(

7.

3)

12425.578

0.001

12(

9.

4)

-

12(

8.

4)

13984.871

12(

5.

7)

-

12(

4.

9)

15965.383

14(10.

4)

-

14(

9.

6)

15549.239

14(

9.

6)

-

14(

8.

6)

13368.777

-0.001

14(

7.

7)

-

14(

6.

9)

12557.658

0.000

14(

6.

8)

-

14(

5.10)

16945.914

O.iXO

7)

-

IS(

9.

14880.287

O.ooO

6(4,3)-

lS(l0,

6(1.5)

7)

-0.001 0.000 0.001

8)

-

16(

7.10)

13594.519

0.001

18(10,

9)

-

18(

9.

13321.066

0.001

lS(

9)

-

18(

8.11)

14633.148

O.CC.0

9)

16(

8.

9.

9)

-

20(10.11)

16128.804

24(12,13)

-

24(11.13)

13102.917

0.000

20(11.

-0.001

26(13,14)

-

26(12,14)

14770.652

O.O!M

31(15.17)

-

31(14.17)

15758.787

0.000

34(16.19)

-

34(15.19)

14584.715

O.OMl

13011.174

O.ooO

37(17.21)

-

a Standard frequency b

Assignment

37(16.21)

deviation

of

less

than

2 kHz from

four

to six

individual

measurements. of

this

transition

confirmed

by MWFT Stark

experiments

MWFT SPECTRUM

253

OF NORBORNADIENE

eluded in Table I. They reach 2 kHz at most with a mean residual error of less than 1 kHz for all transitions. The fitted molecular constants are presented in Table II. We also searched for the rotational spectrum of norbornadiene-2-‘3C (one olefinic carbon of norbomadiene substituted by 13C), the concentration of which is 4.4% in natural abundance. In order to estimate rotational constants of this isotopic species more accurately the calculated constants from the ED structure I in Ref. (3) were corrected using the difference between experimental and calculated constants of the parent norbomadiene. With the help of improved predictions, 12 rotational transitions of norbomadiene-2-13C with J < 10 were assigned. All measured frequencies differed by less than 5 MHz from the above prediction. They are listed in Table III. The correct assignment of the 2(2, 0)-l( 1, 0) transition was again checked by a MWFT Stark experiment. Some transition frequencies could not be determined very accurately because of the presence of many strong transitions of the parent molecule. Only nine transitions were used to fit the three rotational constants. Thereby, all quartic centrifugal TABLE II Rotational Constants (MHz) and Quartic Centrifugal Distortion Constants (kHz) of Norbomadiene and Norbomadiene-2-‘3C This

work

*

strut

ture

I

b

structure

II

b

Norbornadiene A

4273.62.q

B C

4281.2

4315.8

3610.300(1)

3619.8

3675.6

3186.437(l)

3180.3

3135.2

0.311(17)

*J

-0.07256(60)

*JK

0.26841(70)

AK

0.00478(7)

6J

0.092.21(64)

6K Norbarmdiene-2-“C

4249.850(5)

4257.3

4292.2

B

3564.490(5)

3574.1

3628.8

C

3147.211(5)

3141.6

3097.5

deviation

Rotational and

c

A

a Standard b

1)

II.

in parentheses.

constants respectively.

= Centrifugal norbornadiene.

distortion

calculated of

Yokozeki

constants

in units from

of

electron

the

and Kuchitsu constrained

last

digit.

diffraction

structure

(3). to

the values

of

I

254

VOGELSANGER

AND BAUDER

TABLE III Observed Pure Rotational Transitions (MHz) of Norbomadiene-2-“C

J’(K;,K;)-J”(K;.K;)

l(1.0)

-

-0.006 0.003

-

l(1.0)

7(6.2)

-

7(5.2)

7(6.1)

-

7(5.3)

b

9399.918(S)

-0.014

9339.592(4)

0.004

7(5.2)

-

7(4.4)

7754.408(

E(7.2)

-

S(6.2)

11122.871(5)

-0.011

12)

0.046

S(7.1)

-

S(6.3)

11136.054(6)

-0.004

S(4.4)

-

S(3.6)

9143.125(4)

-0.006

g(6.4)

-

g(5.4)

S.539.413(3)

-0.005

5(5,1)

-

5(4.1)

7712.385(100)

c

-0.107

5(5.0)

-

5(4.2)

7763.710(100)

’

-0.003

6(5-l)

-

6(4.3)

7714.828(100)

c

-0.007

ments

’

7814.333(12) 16037.303(10)

Z(2.0)

a Standard

b

O(O.0)

in

deviation

Assignment Not

from

three

to six

individual

frequency

measure-

parentheses.

included

of

this in

the

transition fit.

error

confirmed limits

by WFI’

Stark

experiments.

estimated.

distortion constants were constrained to the values determined for the parent norbornadiene. The fitted rotational constants are included in Table II. The mean residual error for all fitted transitions of 5 kHz is higher than for the parent compound due to the less accurate determination of these weaker transitions. A search for the rotational spectra of the other two r3C monosubstituted isotopes of norbomadiene in natural abundance (2 and 1%) was not successful. We believe that these transitions are too weak to be recognized in the presence of the strong and rich spectrum of the parent molecule. 4. DIPOLE MOMENT

In order to determine the electric dipole moment of norbomadiene, the Stark effects of several low J transitions were measured. A regression analysis of the frequency shifts as a function of the applied static electric field showed a purely quadratic Stark effect for all individual Stark components. Their slopes were used for the determination of the dipole moment in a weighted least-squares fit. The necessary Stark coefficients were calculated using second-order perturbation theory (25). The p. and pb dipole components were constrained to zero in accordance with the Czv symmetry of the molecule. The results are given in Table IV together with the measured Stark slopes.

MWFT

SPECTRUM

255

OF NORBORNADIENE

Attempts to determine also c(=and & led to negative values for their squares with standard deviations comparable to the fitted values. This is further evidence for the C2” symmetry of norbornadiene as well as for the failure to observe pa- or pb-type transitions even when the spectrometer operation was optimized for “nonpolar” transitions. MOLECULAR

STRUCTURE

From the experimentally determined rotational constants of norbornadiene and norbomadiene-2-13C the substitution coordinates of the olefinic carbon Cz in the principal axis system of norbomadiene were calculated using Kraitchman’s equations (26). The following results were obtained: a(C,) = 1.23664(8) A, b(G) = 0.66879( 15) A, and c(G) = 0.50879( 19) A. These coordinates determine directly the bondlength C2 = C3 as well as the nonbonded distance Cz - * - C6 of norbomadiene. The results are listed and compared with ED values in Table V. DISCUSSION

In the present work, the Stark effect was used for the first time in MWFI spectroscopy in order to assist in the initial assignments of a new rotational spectrum. The accidentally small electric dipole moment of norbomadiene was determined accurately

TABLE IV Measured

Stark Slopes and Electric Dipole Moment

J’(K’

-

of Norbomadiene”

Au/E= K’) a’ c

J”(K” K”) a’ c

(HA’-=cm*)

I*1 Obs.

l(1.0)

-

O(O.0)

0

2(2,1)

-

l(1.1)

0

2(2-O)

2(1.1)

-

-

l(1.0)

l(O.1)

pc

a Standard b

deviations

=

CZkb

O.O5S75(16)

0.05890

0.01328(4)

0.01311

1

~.3031(21)

-0.3078

0

-0.01797(5)

-0.01796

1

O.O15S5(4)

0.01586

0

0.01434(6)

0.01426

1

0.3230(10)

0.3260

O.O5S66(9)

in parentheses

D b

in units

of

the

last

digit.

256

VOGELSANGER

AND BAUDER

TABLE V Comparison of Some Distances (A) in Norbomadiene”

This

work

structure

I

b

structure

II

b

Parameter

r G r

= G)

(CyC,)

c (Cz)

d

a Standard

deviation

b From electron ’ d

Values

ra-distance

ra-distance

1.33757(29)

1.3387(12)

1.33x%(11)

2.47328(15)

2.4612

c

2.4622

c

0.50379(19)

0.5084

c

0.4925

=

in parentheses.

diffraction

calculated

Coordinate

rs-distance

c of

from parameters carbon

in units

structures

C,

of of

in principal

of

Yokozeki

Ref. axis

the

last

digit.

and Kuchitsu

(3)

(3). systein.

using the Stark splittings of selected rotational transitions. As demonstrated in Table IV, all measured slopes of seven Stark components deviated by less than four times their standard deviation from the calculated slopes based on the finally adopted dipole moment. This fact indicates that the polarizability anisotropy is negligible for the dipole moment determination of norbornadiene contrary to what was experienced for benzene (16), allene, and allene-1, l-d2 (14). For the latter cases the Stark effect was strongly influenced or even dominated by the polarizability anisotropy. The dipole moments for partially deuterated species or for excited degenerate vibrational states of highly symmetric molecules lie typically in the range of 0.02-0.002 D. These values are considerably smaller than that found for norbornadiene. Conventional Stark-modulated microwave spectrometers are no longer adequate for recording rotational spectra of molecules with dipole moments below 0.2 D because of insufficient modulation. Stark effects are still observed with MWFT spectrometers since higher static electric fields can be maintained than square-wave-modulated fields. The C2 = C3 bondlength compares well with the least-squares results of the ED structure I and II of Yokozeki and Kuchitsu (3) as shown in Table V. However, the nonbonded r, distance between CZ and C6 deviates by more than 0.01 A from that calculated for both ED structures. The latter two differ mainly in the angle CrC&. Since only the CZ nucleus was substituted, the structural parameters of the bridge were not determined from the microwave results. Some indirect information about the rest of the molecular structure is contained in the c coordinate of CZ. The calculated c coordinate of CZ from the parameters of structure I agrees much better with the r, value than that of structure II (3). Rotational constants calculated from structure I deviate by less than 10 MHz from the experimental values whereas those from structure II exhibit differences up to 65 MHz. Thus, structure I from the ED study (3) seems to be more probable consistent with conclusions from NMR spectra (7, 8) and ab initio calculations (9).

MWFT SPECTRUM OF NORBORNADIENE

257

The analysis of the ED data (3) was based on several assumptions regarding the structure in order to reduce the number of structural parameters. The determination of an improved structure which also solves the question about the nonplanarity of the olefinic parts requires the microwave measurements of additional 13Cand D species of norbornadiene. These isotopic species must be synthesized in enriched form because of their low natural abundance. The measurements of their rotational spectra are straightforward after the initial work on the parent and one 13Cspecies. The predicted rotational constants may be estimated reliably taking into account the difference between our results and the values from a model of the parent norbomadiene. ACKNOWLEDGMENTS Financialsupportby the Schweizerischer Nationalfonds(projectNo. 2.005-0.86) is gratefUlly acknowledged. We thankMr. G. Grassifor purifyingthe sampleand ProfessorW. Caminatiand the membersof our group for help and discussions.Specialthanksgo to Dr. M. Rodler for suggesting this study and for many fruitful

discussions. RECEIVED:

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