Conformational landscape, stability, potential energy curves and vibrations of 1,2,3,4 tetrahydroquinoline
TL;DR: In this article, the twisted conformer with the equatorial hydrogen of the NH group is computed to be the most stable conformer of tetrahydroquinoline in S0 and S1 states at various levels of quantum chemical computations.
About: This article is published in Journal of Molecular Structure.The article was published on 2017-05-15. It has received 11 citations till now. The article focuses on the topics: Excited state & Conformational isomerism.
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TL;DR: In this paper, a conformational search for 1, 2, 3, 4-tetrahydroisoquinoline (THIQ) yields a number of conformers in S0 and S1 states.
12 citations
TL;DR: In this article, the global minima in the potential energy surface of 1, 2, 3, 4-tetrahydroisoquinoline (THIQ) have been computed at the ab initio level and DFT methods with M06-2X and ωB97X-D functionals generated consistent results.
8 citations
TL;DR: High-resolution microwave spectroscopy was used to determine the precise molecular structures of the conformers of THQ, and the experimentally evaluated molecular constants unambiguously define the lowest energy conformer of 1,2,3,4-tetrahydroquinoline.
Abstract: The saturated part of the 1,2,3,4-tetrahydroquinoline (THQ) molecule allows for the possibility of multiple conformers' existence. High-resolution microwave spectroscopy, supported by high-level quantum chemistry calculations, was used to determine the precise molecular structures of the conformers of THQ. Via the MP2 calculations, we were able to discriminate four stable conformations, i.e. two pairs of energetically equivalent enantiomorphic conformers. The results of the calculations also indicate that energetically non-equivalent conformers are separated by a low energy barrier (104 cm−1) that allows for conformational cooling to occur. The high resolution rotational spectrum with resolved hyperfine structure in the frequency range of 7–20 GHz was obtained using both the In-phase/quadrature-phase-Modulation Passage-Acquired-Coherence Technique (IMPACT) and the coaxially oriented beam resonator arrangement (COBRA) to perform Fourier transform microwave (FTMW) spectroscopy. The precise values of the rotational constants, 14N nuclear hyperfine coupling parameters and centrifugal distortion parameters were determined from the measured transition frequencies. Based on our experimental results, only the most stable enantiomeric pair of THQ contributes to the rotational spectrum under the conditions of our experiment as the less stable conformers seem to efficiently relax to the lower energy conformers. Thus the experimentally evaluated molecular constants unambiguously define the lowest energy conformer of 1,2,3,4-tetrahydroquinoline.
8 citations
TL;DR: In this article, a twisted conformer with the equatorial orientation of hydrogen atom of both the NH groups and bearing C2 symmetry is computed to be the most stable conformer (C1) in S0.
6 citations
TL;DR: The keto-enol tautomerisation in homophthalic anhydride is investigated in the ground and excited electronic states and the energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.
Abstract: The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S0) and excited (S1) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S0 and enol form with a monocarbonyl structure in S1 indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S1. Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S0 and S1. The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800 cm−1 confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S0. Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π* in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.
6 citations
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TL;DR: Numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, show that density-functional formulas for the correlation energy and correlation potential give correlation energies within a few percent.
Abstract: A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
84,646 citations
TL;DR: In this paper, a detailed study of correlation effects in the oxygen atom was conducted, and it was shown that primitive basis sets of primitive Gaussian functions effectively and efficiently describe correlation effects.
Abstract: In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlof, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects i f the exponents of the functions are optimized in atomic correlated calculations, although the primitive (s p) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations i f the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlof and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of c o r r e l a t i o n c o n s i s t e n t b a s i s s e t s, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.
26,705 citations
TL;DR: In this article, the contracted Gaussian basis sets for molecular calculations are derived from uncontracted (12,8) and ( 12,9) sets for the neutral second row atoms, Z=11-18, and for the negative ions P−, S−, and Cl−.
Abstract: Contracted Gaussian basis sets for molecular calculations are derived from uncontracted (12,8) and (12,9) sets for the neutral second row atoms, Z=11–18, and for the negative ions P−, S−, and Cl−. Calculations on Na...2p63p, 2P and Mg...2p63s3p, 3P are used to derive contracted Gaussian functions to describe the 3p orbital in these atoms, necessary in molecular applications. The derived basis sets range from minimal, through double‐zeta, to the largest set which has a triple‐zeta basis for the 3p orbital, double‐zeta for the remaining. Where necessary to avoid unacceptable energy losses in atomic wave functions expanded in the contracted Gaussians, a given uncontracted Gaussian function is used in two contracted functions. These tabulations provide a hierarchy of basis sets to be used in designing a convergent sequence of molecular computations, and to establish the reliability of the molecular properties under study.
8,079 citations
TL;DR: In this article, a method is presented which utilizes the calculation of the molecular electrostatic potential or the electric field at a discrete number of preselected points to evaluate the environmental effects of a solvent on the properties of a molecular system.
7,618 citations
TL;DR: In this paper, scaling factors for fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach.
Abstract: Scaling factors for obtaining fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach. Semiempirical methods (AM1 and PM3), conventional uncorrelated and correlated ab initio molecular orbital procedures [Hartree−Fock (HF), Moller−Plesset (MP2), and quadratic configuration interaction including single and double substitutions (QCISD)], and several variants of density functional theory (DFT: B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91) have been examined in conjunction with the 3-21G, 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), and 6-311G(df,p) basis sets. The scaling factors for the theoretical harmonic vibrational frequencies were determined by a comparison with the corresponding experimental fundamentals utilizing a total of 1066 individual vibrations. Scaling factors suitable for low-frequency vib...
6,287 citations