Uncertainties in Scaling Factors for ab Initio Vibrational Frequencies

Irikura, Karl K.; Johnson, Russell D.; Kacker, Raghu N.

doi:10.1021/jp052793n

Cited by 462 publications

(419 citation statements)

References 35 publications

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“…This is probably due both to deficiencies in the electronic-structure level of theory used and to the neglect of anharmonicity, since the harmonic approximation is inherent in the normal-mode calculations. Thus, as listed in Table 2, we scale the computed wavenumbers by multiplication by a scaling factor c (0.983), which was obtained from a least-squares fit corresponding to the following formula 67 :…”

Section: Computational Detailsmentioning

confidence: 99%

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

Liu

Ojamäe

2014

J. Phys. Chem. A

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Abstract:The presence of specific hydrocarbon gas molecules in various types of water cavities in natural gas hydrates (NGHs) are governed by the relative stabilities of these encapsulated guest molecule ─ water cavity combinations. Using molecular quantum-chemical dispersion-corrected hybrid density functional computations, the interaction energies (ΔEhost-guest) and cohesive energies (ΔEcoh), enthalpies and Gibbs free energies for the complexes of host water cages and hydrocarbon guest molecules are calculated at the ωB97X-D/6-311++G(2d,2p) level of theory. The zero point energy effect of ΔEhost-guest and ΔEcoh is found to be quite substantial. The energetically optimal host-guest combinations for seven hydrocarbon gas molecules (CH4, C2H6, C3H6, C3H8, C4H8, i-C4H10, and n-C4H10) and various water cavities (D, ID, T, P, H and I) in NGHs are found to be CH4@D, C2H6@T, C3H6@T, C3H8@T, C4H8@T/P/H, i-C4H10@H, and n-C4H10@H, as the largest cohesive energy magnitudes will be obtained with these host-guest combinations. The stabilities of various water cavities enclosing hydrocarbon molecules are evaluated from the computed cohesive Gibbs free energies: CH4 prefer to be trapped in a ID cage; C2H6 prefer T cages; C3H6 and C3H8 prefer T and H cages; C4H8 and i-C4H10 prefer H cages; and n-C4H10 prefer I cages. The vibrational frequencies and Raman intensities of the C-C stretching vibrational modes for these seven hydrocarbon molecules enclosed in each water cavity are computed. A blue shift results after the guest molecule is trapped from gas phase into various water cages due to the host-guest interactions between the water cage and hydrocarbon molecule. The frequency shifts to the red as the radius of water cages increases. The model calculations support the view that C-C stretching vibrations of hydrocarbon molecules in the water cavities can be used as a tool to identify the types of crystal phases and guest molecules in NGHs.2

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Section: Computational Detailsmentioning

confidence: 99%

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

Liu

Ojamäe

2014

J. Phys. Chem. A

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“…The B3LYP functional and the 6-31G (d) basis sets were used and the optimizations were unconstrained. Vibrational frequencies of the normal modes were scaled by the standard scaling factor 0.96 [31].…”

Section: Analysis and Characterizationmentioning

confidence: 99%

The effect of halloysite modification combined with in situ matrix modifications on the structure and properties of polypropylene/halloysite nanocomposites

Khünová¹,

Kristóf²,

Kelnar³

et al. 2013

Express Polym. Lett.

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The effect of various modifications/intercalations of halloysite and the combination of these modifications with in situ PP matrix modification was investigated with respect to the structure and properties of the polypropylene/halloysite nanocomposites. Hexadecyl-tri-methyl-ammonium-bromide (HEDA), 3-aminopropyltrimethoxysilane and urea were used as the intercalators/modifiers. The best intercalation was found for urea, although an unexpected insignificant impact on the mechanical properties also resulted as a consequence of the urea polarity and the significant decrease in PP crystallinity. However, the simultaneous application of 4,4!-diphenylmethylene dimaleinimide (DBMI) brought about an increase in the mechanical behavior by increasing the halloysite/PP affinity as a result of in situ matrix modification. This effect was further supported by coupling between the PP and halloysite (HNT) in the system containing urea-intercalated HNT. This can be explained by the occurrence of a urea-supported reaction between the imide ring of DBMI and the OH groups of the HNT, which resulted in the best mechanical behaviors achieved in this study

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“…Recently, discussions on the uncertainties in scaling factors for vibrational frequencies indicated that the values of the calculated frequencies can only have two significant figures. 40 Thus, the uncertainty of the individual frequency in the 2800-3000 cm -1 region can be as big as more than 50 cm -1 . Therefore, the theoretical results are shown here just for qualitative comparison with experiment and to interpret the spectra, and the sequence between the calculated overtone frequency of the bending mode and the fundamental stretching mode should not be taken literally.…”

Section: Theoretical Calculations Of Raman Spectra Of Gaseous Ethanolmentioning

confidence: 99%

New C−H Stretching Vibrational Spectral Features in the Raman Spectra of Gaseous and Liquid Ethanol

et al. 2007

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Traditionally, the Raman spectrum of ethanol in the C-H vibrational stretching region between 2800 cm -1 and 3100 cm -1 had been assigned as three bands of the -CH 2 symmetric stretching, the -CH 3 symmetric stretching, and the -CH 3 antisymmetric stretching. In this report, new Raman spectral features were observed for ethanol and two deuterated ethanols, that is, CD 3 CH 2 OH and CH 3 CD 2 OH, in both gaseous and liquid phases with polarized photoacoustic Raman spectroscopy (PARS) as well as normal Raman spectroscopy. With the aid of depolarization ratio measurements and quantum chemical calculations, different assignments were presented to the complex spectral features in the ethanol Raman spectra. In the gaseous ethanol spectra, the band at ∼2882 cm -1 was assigned to the overlapping symmetric stretching vibrational modes of both -CH 2 and -CH 3 ; the band at ∼2938 cm -1 was assigned to the two symmetric -CH 3 Fermi resonance modes and the weak -CH 2 antisymmetric stretching mode; and the band at ∼2983 cm -1 was assigned to the symmetric -CH 2 Fermi resonance mode and the weak -CH 3 antisymmetric stretching mode. The liquid ethanol spectral features are similar to the gaseous spectra with a much stronger -CH 3 antisymmetric stretching mode and a red shift of about 10 cm -1 , which can be attributed to the effects of solvent interactions. The new assignments of both the gaseous and liquid ethanol spectra not only confirmed the recent results from the sum-frequency generation vibrational spectroscopy studies of the ethanol molecules at the air/liquid interface but the differences in the gaseous and liquid phases, as well as at the interfaces, can also provide detailed experimental evidence in understanding of the molecular interactions and dynamics of the ethanol molecule in different chemical environments.

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Uncertainties in Scaling Factors for ab Initio Vibrational Frequencies

Cited by 462 publications

References 35 publications

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

The effect of halloysite modification combined with in situ matrix modifications on the structure and properties of polypropylene/halloysite nanocomposites

New C−H Stretching Vibrational Spectral Features in the Raman Spectra of Gaseous and Liquid Ethanol

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