Internal rotation arena: Program performances on the low barrier problem of 4-methylacetophenone

Abstract

In the rotational spectroscopy community, several popular codes are available to treat multiple internal rotors in a molecule. In terms of the pros and the cons of each code, it often is a difficult task to decide which program to apply to a specific internal rotation problem. We faced this issue when dealing with the spectroscopic fingerprint of 4-methylacetophenone (4MAP), recently investigated in the microwave region, which we here extend into the millimeterwave region. The methyl group attached to the phenyl ring in 4MAP undergoes internal rotation with a very low barrier of only 22 cm 1 . The acetyl methyl group features a much higher barrier of about 580 cm 1 . The performances of a program using the so-called "local" approach in terms of Herschbach's perturbative treatment, SPFIT, as well as three programs XIAM, ERHAM, and ntop, representing "global" fits, were tested. The results aim at helping spectroscopists in the decision on how to tackle their own internal rotation problems. 65 Pickett's Calpgm suite of programs [17]. None of these is commonly applied in the treatment of multiple rotors. Since the Hamiltonian is defined in the  axis system, the meaning of fitted parameters is often not as clear to the users, who are usually more familiar with the interpretation in the principal axis system. The codes are very 70 flexible and a large number of fit parameters are available [10,16].CAM: An approach combining the convenience of the PAM with the benefits of the RAM is the so-called Combined Axis Method (CAM) [18,19], as used in the program XIAM [5]. In a first step, the internal rotation is treated in the  axis system, and subsequently, a 75 rotation matrix is used for a transformation into the principal axis system. An advantage of this method is the clear physical meaning

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

“…For p-tolualdehyde (2), a V 3 potential of 28 cm −1 with a V 6 contribution of −4.8 cm −1 [155] is reported. In the investigations of 4-methylacetophenone (8) [77,160], the value of approximately 22 cm −1 of the p-methyl rotor is close to the value of 18 cm −1 observed for p-cresol (molecule (4) in Figure 5) [145] and 28 cm −1 for p-tolualdehyde (2), but much lower than the value of 50 cm −1 of p-methylanisole (molecule (11) in Figure 5) [151]. If substituting the para position of toluene breaks the C 2v symmetry of toluene less effectively in terms of symmetry-breaking electronic contributions, then a smaller V 3 contribution is obtained.…”

“…Barriers to methyl internal rotation in toluene derivatives with a carbonyl-containing substituent (in cm −1 ). (1) m-Tolualdehyde [154], (2) p-tolualdehyde [155], (3) o-toluic acid [156], (4) m-toluic acid [157], (5) p-toluic acid [65], (6) acetophenone[158], (7) acetovanillone[159], (8) 4-methylacetophenone[77,160], (9) 6-hydroxy-3-methoxyacetophenone[159].…”

Potential and kinetic interaction of two internal tops in molecules belonging to C2v(M) molecular symmetry group

“…Internal rotation in molecules (one of the types of large amplitude motion (LAM)) continues to attract the attention of researchers today [1][2][3][4][5][6][7]. This is due to the important role that internal rotation plays in determining the thermodynamic properties of molecules [1], as well as in the formation of the fine structure of rotational and vibrational-rotational absorption bands [8][9][10].…”