U.S. flag

An official website of the United States government, Department of Justice.

Correlation of Vapor Phase Infrared Spectra and Regioisomeric Structure in Synthetic cannabinoids

NCJ Number
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy Volume: 196 Issue: 375 Dated: 2018 Pages: 384
Date Published
10 pages

This article describes the vapor phase infrared spectra for 12 regioisomeric indoles.


The twelve 1-n-pentyl-2-, 3-, 4-, 5-, 6- and 7-(1- and 2-naphthoyl)-indoles each have the same substituents attached to the indole ring, identical elemental composition (C24H23NO) yielding identical nominal and accurate masses. These 12 isomers cover all possible positions of carbonyl bridge substitution for both indole (positons 2-7) and naphthalene rings (positions 1 and 2). Regioisomeric compounds can represent significant challenges for mass based analytical methods; however, infrared spectroscopy is a powerful tool for the identification of positional isomers in organic compounds. The vapor phase infrared spectra of these 12 uniquely similar compounds were evaluated in GC-IR experiments. These spectra show the bridge position on the indole ring is a dominating influence over the carbonyl absorption frequency observed for these compounds. Substitution on the pyrrole moiety of the indole ring yields the lowest Cdouble bondO frequency values for position 2 and 3, giving a narrow range from 1656 to 1654 cm-1. Carbonyl absorption frequencies are higher when the naphthoyl group is attached to the benzene portion of the indole ring, yielding absorption values from 1674 to 1671 cm-1. The aliphatic stretching bands in the 2900 cm22-1 region yield a consistent triplet pattern because the N-alkyl substituent tail group remains unchanged for all twelve regioisomers. The asymmetric CH2 stretch is the most intense of these three bands. Changes in positional bonding for both the indole and naphthalene ring systems results in unique patterns within the 700 wavenumber out-of-plane region, and these absorption bands are different for all 12 regioisomers. (publisher abstract modified)

Date Published: January 1, 2018