Issue 36, 2024

Fulminic acid: a quasibent spectacle

Abstract

Fulminic acid (HCNO) played a critical role in the early development of organic chemistry, and chemists have sought to discern the structure and characteristics of this molecule and its isomers for over 200 years. The mercurial nature of the extremely flat H–C–N bending potential of fulminic acid, with a nearly vanishing harmonic vibrational frequency at linearity, remains enigmatic and refractory to electronic structure theory, as dramatic variation with both orbital basis set and electron correlation method is witnessed. To solve this problem using rigorous electronic wavefunction methods, we have employed focal point analyses (FPA) to ascertain the ab initio limit of optimized linear and bent geometries, corresponding vibrational frequencies, and the HCN + O(3P) → HCNO reaction energy. Electron correlation treatments as extensive as CCSDT(Q), CCSDTQ(P), and even CCSDTQP(H) were employed, and complete basis set (CBS) extrapolations were performed using the cc-pCVXZ (X = 4–6) basis sets. Core electron correlation, scalar relativistic effects (MVD1), and diagonal Born–Oppenheimer corrections (DBOC) were all included and found to contribute significantly in determining whether vibrationless HCNO is linear or bent. At the all-electron (AE) CCSD(T)/CBS level, HCNO is a linear molecule with ω5(H–C–N bend) = 120 cm−1. However, composite AE-CCSDT(Q)/CBS computations give an imaginary frequency (51i cm−1) at the linear optimized geometry, leading to an equilibrium structure with an H–C–N angle of 173.9°. Finally, at the AE-CCSDTQ(P)/CBS level, HCNO is once again linear with ω5 = 45 cm−1, and inclusion of both MVD1 and DBOC effects yields ω5 = 32 cm−1. A host of other topics has also been investigated for fulminic acid, including the dependence of re and ωi predictions on a variety of CBS extrapolation formulas, the question of multireference character, the N–O bond energy and enthalpy of formation, and issues that give rise to the quasibent appellation.

Graphical abstract: Fulminic acid: a quasibent spectacle

Supplementary files

Article information

Article type
Paper
Submitted
08 Jul 2024
Accepted
19 Aug 2024
First published
28 Aug 2024
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2024,26, 24109-24125

Fulminic acid: a quasibent spectacle

A. M. Allen, L. N. Olive, P. A. Gonzalez Franco, S. R. Barua, W. D. Allen and H. F. Schaefer, Phys. Chem. Chem. Phys., 2024, 26, 24109 DOI: 10.1039/D4CP02700K

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