Application of Laser-Excited Raman Spectroscopy to Organic Chemistry. II. Conformers of some Isochromans

Stanley K. Freeman International Flavors & Fragrances Inc., Research and Development Center, Union Beach, New Jersey

(Received 9 June 1969; revision received 25 July 1969)

R a m a n spec t ra of 4-methyl i soehroman and i ts a romat ic r ing s u b s t i t u t e d der iva t ives ind ica te the presence o f two confo rmers in the l iquid s ta te and only one in the solid, while the 1- a n d 3-methyl ana log a s sume one con fo rma t i on only in both s ta tes . The presence of a me thy l g roup a d j a c e n t to the r i ng oxygen a tom s ter ieal ly p reven t s adopt ion of one of the two pos- sible con fo rma t ions due to 1,3-interact ion. No such res t r ic t ion is imposed on the 4-methyl compounds .

I N D E X ~:~EADINGS : Conforma t ion ; l~aman spec t ra ; I soehromans .

INTRODUCTION

The Raman effect has been employed as a probe for detecting conformational isomers in a var ie ty of or- ganic compounds. 1-3 The presence of a band(s ) in the spect rum of the liquid but not in the solid fo rm of a compound is good evidence that a second con- fo rmer is in equilibrium with the conformer present in the solid. This paper reports the first Ramau s tudy on such conformers of isoehromans.

I n t e n s i t y

liquid

solid

7 1 0 6 8 2 cm-I

l~IG. 1. P a r t i a l R a m a n spec t ra of I .

l iquid--682~ 703 c m -1

so l id - -682 cm "a

I

l iquid and so l id - -702 cm -~

I I

l iquid and so l id - -685 em "4

I I I

l iquid a n d so l id - -683 era- '

I V

l iqu id- -710 , 730 cm -~ so l id - -710 cm -~

V

732 cm -1 716 cm -1 740 cm -1 710, 732 cm -1

VI VII VIII IX

I. EXPERIMENTAL

Spectra were recorded on a Spex Industr ies, Inc. laser Raman system equipped with a ~-m Czerny- Turne r double monochromator (1200 g / m m blazed at 5000 A, and a Spex ER-1 (picoammeter) electronic readout assembly. A Spec t ra -Phys ics H e - N e 6328 A laser was operated at approx imate ly 75 roW, and an

42 Volume 24, Number 1, 1970 APPLIED SPECTROSCOPY

ITT Ind. Co. F W 130 photomultiplier tube was main- tained between 1600-1850 ¥ (10-8-10 -9 A full scale). Frequencies reported in this paper are accurate to ± 3 cm -1 and the spectrometer slits were set for 5- cm -z resolution at the exciting line.

Spectra were obtained on liquid and solid samples (GLC purities > 90%) contained ill l-ram bore boro- silicate capillary tubing. 4 Immediately subsequent to generating a spectrum of a solid material, the sub- stance was melted and a Raman curve obtained on the liquid in the same capillary.

II. RESULTS AND DISCUSSION

The l~aman spectra of solid and liquid 3,4,6,7,8, 9-hexahydro-4,6,6,9,9-pentamethyl-lH-naphtho [2,3-c] pyran (I) differ by the presence of a band in the spectrmn of the liquid which is absent in the spec- t rum of the solid (see Fig. 1). By contrast, the spec- tra of II and ni as well as of the apo-methyl com- pound, IV, show no differences in this region between the liquid and solid states of the compounds. De- polarization values of approximately 0.03 were ob- tained on these bands for all four materials in the liquid state.

The Raman effect for the liquid form of ],3,4,6, 7,8-hexahydro-4,6,6,7,8,8-hexamethyl-eyclopenta [g]-2- benzopyran (an approximately equal mixture of con- figurational isomers with reference to the 4- and 7-

methyl groups) (v), also shows two bands at circa 700 cm -1 (p = 0.03), while one band only is observed for the solid state.

This general spectral region in isoehroman (vI), 1- methyl isochroman (vii), and 3-methyl isoehroman (VIII), all as liquids, shows a single band for each substance (p = approximately 0.03). Oll the other hand, two bands were evident for 4-methyl isochro- man (Ix) (p = 0.02). The locations, relatively high scattering efficieneies, and low depolarization values of these emissions for I to Ix are consonant with a r ing breathing mode of the d ihydropyran moiety.

The data indicate that a methyl group substituted adjacent to the oxygen atom in isochroman deriva- tives sterically prevents adoption of one of the two possible conformations due to 1,3-interaction. No such restriction is imposed on the 4-methyl compounds; the oxygen atom is free to assume both cis and trcbns positions with relation to the 4-methyl group. In the solid state, the methyl-substituted dihydropyran ring expectedly is stabilized in one conformation.

1. F. E. 1Vialherbe and H. J. Bernsteln, 5. Am. Chem. Soc. 74, 4408 (1952).

2. ~. L. Allinger and L. A. Freiberg, 5. Am. Chem. Soe. 83, 5028 (1961).

3. M. Hayashi, S. Shiro, T. Oshima and Murata, ~., Bull. Chem. Soc. Japan 39, 118 (1966).

4. S. K. Freeman and D. L. Landon, Anal. Chem. 42, 398 (1969).

APPLIED SPECTROSCOPY 43