March 2023

Pyrrole, c-C4H5N, has been the subject of several unsuccessful astronomical searches separated by more than four decades. The first searches were both reported in 1980. Myers and coworkers sought pyrrole toward Sgr B2, while Kutner and coworkers sought pyrrole toward both Sgr B2 and TMC-1. Both searches used the 43m telescope at Green Bank, West Virginia. After four decades passed with no other reported searches, Barnum and coworkers made a new search for pyrrole — and about 20 other heterocyclic compounds — toward TMC-1 with the 100m Green Bank Telescope. Even with the superior sensitivity of the newer telescope, pyrrole defied detection. Newer data about the rotational spectrum of pyrrole were also available: the 1980 searches were based on 1969 lab work by Nygaard et al.; the recent search also used measurements by Wlodarczak et al published in 1988.

Researcher Links
TJ Barnum
MA Siebert
RA Loomis
PB Changala
SB Charnley
ML Sita
C Xue
AJ Remijan
AM Burkhardt
BA McGuire
IR Cooke

While nitrogen atoms are often found to adopt a pyramidal bonding structure—as in ammonia, cyanamide, hydroxylamine, methylamine, vinylamine, and aminoacetonitrile— it can also be planar, as it is in pyrrole and formamide.

The pyramidal form occurs because nitrogen has three singly occupied 2p orbitals that are 90° apart; the bond angles expand from the nominal value due to bond pair repulsion. The planar form is related to the trigonal planar form of the CH3 radical with sp2 hybridization of the three bonds pairs. In contrast to CH3, planar NH3 has a doubly occupied 2p orbital on nitrogen. The planar form tends to be favored when the 2p2 lone pair can undergo delocalized π bonding with neighboring atoms, as it does in pyrrole.

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