Accurate ab initio quartic force fields of cyclic and bent HC2N isomers
Natalia Inostroza1, Xinchuan Huang, Timothy J Lee
1NASA Ames Research Center, Moffett Field, California 94035-1000, USA. Natalia.p.Inostrozapino@nasa.gov
The Journal of Chemical Physics
|January 10, 2012
Summary
This study calculates spectroscopic parameters for three HC(2)N isomers using quartic force fields. Accurate constants were determined for cyclic and bent singlet isomers, aiding astronomical and laboratory analysis.
Area of Science:
- Computational chemistry
- Theoretical spectroscopy
- Astrochemistry
Background:
- Understanding the properties of small carbon-nitrogen molecules like HC(2)N is crucial for astrochemistry.
- Accurate spectroscopic data is needed for identifying and characterizing molecules in interstellar space.
Purpose of the Study:
- To calculate equilibrium structures and predict spectroscopic parameters for three HC(2)N isomers.
- To assess the accuracy of quartic force fields (QFFs) combined with perturbation theory (PT) and variational methods for predicting molecular properties.
- To provide highly accurate spectroscopic constants for HC(2)N isomers for potential astronomical and laboratory identification.
Main Methods:
- Highly correlated ab initio quartic force fields (QFFs) were developed.
- Coupled-cluster calculations with singles and doubles and perturbational triples (CCSD(T)) were employed.
- Basis sets up to cc-pV5Z were used, with extrapolation to the basis set limit.
- Core-correlation and scalar relativistic corrections were included.
- The nuclear Schrödinger equation was solved using second-order perturbation theory (PT) and variational methods.
Main Results:
- Accurate fundamental vibrational frequencies and spectroscopic constants were obtained for the cyclic and bent singlet HC(2)N isomers.
- The quasilinear triplet isomer's bending motion proved challenging for QFFs, though some constants were well-reproduced.
- Significant dipole moments were calculated for all three isomers (3.05 D, 3.06 D, and 1.71 D).
Conclusions:
- The combination of QFFs with PT or variational methods yields highly accurate spectroscopic constants for the cyclic and bent singlet HC(2)N isomers.
- The determined spectroscopic constants are the most accurate available and are expected to aid in the interpretation of astronomical observations and laboratory experiments.
- QFFs alone may not fully capture the complexities of quasilinear isomers' vibrational spectra.
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