Periodic potential functions for pseudorotation and internal rotation

We demonstrate the application of molecular rotational spectroscopy to measure the conformation isomerization rate of vibrationally excited pent-1-en-4-yne (pentenyne). The rotational spectra of single quantum states of pentenyne are acquired by using a combination of IR-Fourier transform microwave double-resonance spectroscopy and high-resolution, single-photon IR spectroscopy. The quantum states probed in these experiments have energy eigenvalues of Ϸ3,330 cm ؊1 and lie above the barrier to conformational isomerization. At this energy, the presence of intramolecular vibrational energy redistribution (IVR) is indicated through the extensive local perturbations found in the high-resolution rotation-vibration spectrum of the acetylenic C-H stretch normalmode fundamental. The fact that the IVR process produces isomerization is deduced through a qualitatively different appearance of the excited-state rotational spectra compared with the pure rotational spectra of pentenyne. The rotational spectra of the vibrationally excited molecular eigenstates display coalescence between the characteristic rotational frequencies of the stable cis and skew conformations of the molecule. This coalescence is observed for quantum states prepared from laser excitation originating in the ground vibrational state of either of the two stable conformers. Experimental isomerization rates are extracted by using a threestate Bloch model of the dynamic rotational spectra that includes the effects of chemical exchange between the stable conformations. The time scale for the conformational isomerization rate of pentenyne at total energy of 3,330 cm ؊1 is Ϸ25 ps and is 50 times slower than the microcanonical isomerization rate predicted by the statistical Rice-Ramsperger-Kassel-Marcus theory.IVR ͉ microwave ͉ infrared ͉ double-resonance R otational spectroscopy is one of the most powerful tools in physical chemistry for the determination of gas-phase molecular structure (1). The development of molecular beam spectrometers for rotational spectroscopy-most notably the Fourier-transform microwave (FTMW) spectroscopy technique introduced by Balle and Flygare (2)-has greatly expanded the range of chemical systems that can be studied. Rotational spectroscopy can be routinely used to study challenging structural problems such as the conformations of large molecules (3), weakly bound molecular complexes (4), and radicals (5). In addition, rotational spectroscopy of low-energy vibrational and torsional levels can be used to determine accurate potential energy surfaces for large amplitude motion in ''floppy'' systems (1). Here we demonstrate a fundamentally unique application of rotational spectroscopy. We show that isomerization kinetics can be determined from the rotational spectrum of a highly vibrationally excited molecule.The physical basis of pure rotational spectroscopy is that when the total angular momentum is conserved, the rotational frequency is directly related to the principal moments of inertia of the molecule and, therefore, to its struct...

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“…The probability distributions for the lowest energy onedimensional (1D) torsional level of each conformer (27) are shown in Fig. 1.…”

Section: Resultsmentioning

confidence: 99%

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm ⁻¹ of internal energy measured by dynamic rotational spectroscopy

Dian

Brown

Douglass

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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“…Following the method of Lewis et al (13), the Hamiltonian matrix corresponding to eq. [I] was set up in terms of sine and cosine basis functions and diagonalized to yield the eigenvalues and wavefunctions.…”

Section: Discussionmentioning

confidence: 99%

The conformational changes accompanying the triplet–singlet electronic excitation of acetaldehyde, CH₃CHO

Moule¹,

Ng²

1985

Can. J. Chem.

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The first electronic absorption system of acetaldehyde was recorded in the vapour phase at room temperature. The many-banded spectrum proved to be very complex and it was only at the extreme red edge of the absorption that the pattern became simple enough to analyze. The major experimental requirement was a high pressure × path length (500 Torr × 168 m). Spectra were interpreted in terms of the torsional mode ν′15 and ν″15 attached to the ν′14 out-of-plane bending mode. Both modes were highly Franck–Condon active and the [Formula: see text] band at 27240.4 cm−1 was not directly observed. The barrier to rotation of the methyl group was 618.5 cm−1. More surprising was the observation that the methyl group undergoes a rotation from a [Formula: see text] eclipsed configuration to a staggered configuration. The intensity of the ν′14 quantum addition and its position in the spectrum suggest that the aldehydic hydrogen is nonplanar in the ã state.

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“…13. Puckered conformations were generated by using the B, ring-puckering geometrical model that will be described elsewhere (14).…”

Section: Discussionmentioning

confidence: 99%

Complete analysis of the ¹H nmr spectrum of tetrahydrothiophene

Esteban¹

1980

Can. J. Chem.

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ANGEL LUIS ESTEBAN and ERNESTO D~Ez. Can. J. Chem. 58,2340Chem. 58, (1980. The 'H nmr spectrum of tetrahydrothiophene has been analyzed. The best set of coupling values (Hz) is: ,JA -,JB = 2.18, 3Ji = 5.53, 3Jfi = 7.31, ' J i e = 6.80, 3J19B = 6.37, 4 J i = -0.32, 4J6 = 0.25, 4JXB = 0.26, 4Jj,B = -0.47(A and B refer toa-and P-protons and c and t denote respectively the same and opposite sides of the ring). The variances observed between the values of this set and those reported by Lozach et al., particularly the interchange of the 4Ji and 4J6 couplings, are discussed and it is concluded that our set is the one that reproduces the most fine details of the spectrum profile. The proposed set has been selected as being the most physically meaningful, from amongst 24 equally valid sets of couplings, on the basis of a better agreement with the 3JB and 4JAB values calculated from a Karplus relationship and from the INDO-FPT method respectively. In these calculations it was assumed that the molecule was interconverting rapidly (on the nmr time scale) between two puckered conformations with C , symmetry. A detailed discussion of the dependence of the couplings on the height of the pseudorotational barrier and on the temperature indicate that the minimum values of the potential energy correspond to these conformations. Dans ces calculs, on afait I'hypothkse que la molCcule subit rapidement une interconversion (a I'echelle du temps de la rmn) entre deux conformations gauches avec une symetrie C , . Une discussion detaillee de I'effet de la hauteur de la barrikre pseudo-rotationnelle et de la temperature sur les couplages, indique que les valeurs minima de I'tnergie potentielle correspondent a ces conformations.[Traduit par le journal]

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Periodic potential functions for pseudorotation and internal rotation

Cited by 393 publications

References 30 publications

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm ⁻¹ of internal energy measured by dynamic rotational spectroscopy

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm ⁻¹ of internal energy measured by dynamic rotational spectroscopy

The conformational changes accompanying the triplet–singlet electronic excitation of acetaldehyde, CH₃CHO

Complete analysis of the ¹H nmr spectrum of tetrahydrothiophene

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Periodic potential functions for pseudorotation and internal rotation

Cited by 393 publications

References 30 publications

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm −1 of internal energy measured by dynamic rotational spectroscopy

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm −1 of internal energy measured by dynamic rotational spectroscopy

The conformational changes accompanying the triplet–singlet electronic excitation of acetaldehyde, CH3CHO

Complete analysis of the 1H nmr spectrum of tetrahydrothiophene

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Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm ⁻¹ of internal energy measured by dynamic rotational spectroscopy

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm ⁻¹ of internal energy measured by dynamic rotational spectroscopy

The conformational changes accompanying the triplet–singlet electronic excitation of acetaldehyde, CH₃CHO

Complete analysis of the ¹H nmr spectrum of tetrahydrothiophene