Neopentane Vibrations: High Resolution Spectra and Anharmonic Calculations
Peter F Bernath1, Edwin L Sibert Iii2, Michael Dulick1
1Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, United States.
The Journal of Physical Chemistry. A
|April 8, 2020
Summary
High-resolution infrared spectra of neopentane (C5H12) were analyzed at different temperatures. Quantum chemical calculations revealed couplings between methyl groups influencing vibrational transitions.
Area of Science:
- Molecular spectroscopy
- Quantum chemistry
- Vibrational analysis
Background:
- Neopentane (2,2-dimethylpropane, C5H12) is a highly symmetric spherical top molecule (Td symmetry).
- Understanding its vibrational modes is crucial for detailed theoretical and experimental comparisons.
- High-resolution spectroscopy at reduced temperatures enhances spectral clarity.
Purpose of the Study:
- To record and analyze high-resolution infrared absorption spectra of neopentane.
- To compare experimental data with quantum chemical calculations for vibrational assignments.
- To investigate couplings between methyl groups and their effect on vibrational transitions.
Main Methods:
- High-resolution infrared absorption spectroscopy at room temperature and 232 K.
- Quantum chemical calculations using B3LYP/6-311++(d,p)/VPT2 and harmonic CCSD(T)-pVTZ methods.
- Analysis in both normal and local mode representations.
Main Results:
- Observed five infrared absorption bands, four strong (t2 modes) and one weak, in the mid-infrared region.
- Identified characteristic rotational structures for t2 modes.
- Quantum chemical calculations aided vibrational assignments and elucidated couplings.
- Highlighted significant couplings between neighboring methyl groups influencing observed transitions.
Conclusions:
- The study provides a detailed analysis of neopentane's fundamental vibrations.
- Local mode representation effectively highlights methyl group couplings.
- Discrepancies between theory and experiment were analyzed, identifying specific couplings responsible for differences.
More Related Videos
Related Concept Videos
01:20¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)
1.5K
When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
1.5K
01:16IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration
2.6K
A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to...
According to Hooke's law, the vibrational frequency is directly proportional to...
2.6K
00:52¹H NMR of Conformationally Flexible Molecules: Temporal Resolution
1.2K
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
1.2K
01:08IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations
1.6K
Identical bonds within a polyatomic group can stretch symmetrically (in-phase) or asymmetrically (out-of-phase). Similar to hydrogen bonding, these vibrations also influence the shape of the IR peak. Generally, asymmetric stretching frequencies are higher than symmetric stretching frequencies. For example, primary amines exhibit two distinct IR peaks between 3300–3500 cm−1 corresponding to the symmetric and asymmetric N-H stretching, while secondary amines exhibit a single...
1.6K
01:24IR Spectroscopy: Molecular Vibration Overview
4.3K
When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamental vibrational motions that result in infrared absorption can be classified as stretching or bending vibrations.
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...
4.3K
01:14NMR Spectroscopy of Aromatic Compounds
6.0K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
6.0K