Far-Infrared Spectrum, ab Initio, and DFT Calculations and Two-Dimensional Torsional Potential Function of Dimethylallene (3-Methyl-1,2-butadiene)

Abstract

The far-infrared and infrared spectra of gaseous dimethylallene (3-methyl-1,2-butadiene), (CH3)2CCCH2, were measured as well as the Raman spectra of the compound in all three physical phases. The spectra were assigned using infrared band contours, Raman depolarization ratios and ab initio predicted harmonic frequencies and intensities as criteria. The torsional energy level splittings were determined for the ground state and one torsional excited state from previously published microwave spectra by fitting the frequencies to an effective rotational Hamiltonian for molecules with two periodic large-amplitude motions. The splittings and the data from the far-infrared spectrum were used to derive the two-dimensional torsional potential function of dimethylallene. This function with an effective barrier V 3 eff = 726(4) cm-1 is compared with the results from the microwave analysis and with the potential function calculated by a number of ab initio methods (among them B3LYP/6-31G(d) and MP2/DZ(d)).

The Millimeter‐ and Submillimeter‐Wave Spectrum of13C1‐Methyl Formate (H13COOCH3) in the Ground State

“…Known by the acronym ERHAM, the procedure was developed for a molecule with one or two internal rotors by Groner and collaborators (Groner 1992(Groner , 1997Groner et al 1998), and has been successfully applied to a number of molecules in their ground and excited torsional states, such as dimethyl ether (Groner et al 1998), 3-methyl-1,2-butadiene (Bell et al 2000), acetone (Groner et al 2002, ethyl methyl ether (Fuchs et al 2003), dimethyl diselenide (Groner et al 2004), and acetone-13 C (Lovas & Groner 2006 (Groner et al 2007), which is the NH 2 analog of methyl formate.…”

Analysis and fit of the Fourier-transform microwave spectrum of the two-top molecule N-methylacetamide

“…Even though at least two alternative two-top programs are used in the literature [7][8][9][10][11][12][13][14][15][16][17], we decided to write our own (as described briefly below) in order to better understand the model and the physical and numerical approximations being used.…”

“…The first involves molecules with two identical tops, so that the group theory, torsion-rotation Hamiltonian, and splitting patterns are somewhat different from those treated here. For recent examples of such studies, see [7][8][9][10][11][12][13][14][15][16]. For recent examples of the second group, which involves molecules with two inequivalent methyl tops like that studied here, see [17].…”

Microwave structural studies of organoselenium compounds 1. Microwave spectra, molecular structure, and methyl barrier to internal rotation of dimethyl diselenide

“…The computer program used earlier for cases like acetone [24][25][26][27] was modified to treat all cases of equal or lower symmetry. It sets up and diagonalizes the Hamiltonian matrix described above.…”

“…Although similar to the extensions of WoodsÕ approach [21][22][23], it is much more general because: (i) it is not restricted to a specific vibrational state, (ii) it applies one set of internal rotation parameters to several vibrational states but allows for separate spectroscopic parameters for these states, (iii) it includes centrifugal distortion, and (iv) it allows for tunneling contributions to rotational and centrifugal distortion ''constants.'' It has been applied successfully to the spectra of dimethyl ether [25], 3-methyl-1,2-butadiene [26], acetone [27], and ethyl methyl ether [28] with fits to experimental precision for more than 1000 frequencies.…”