Magnetic vibrational circular dichroism of methyl halides in solution

Abstract

The magnetic vibrational circular dichroism of four methyl halides in CCl4 solution has been measured. Contrary to expectations based on available theoretical models for MVCD, predominantly B-term MVCD is found for both the A1 and E C–H stretching vibrations. MVCD of these two modes are opposite in sign for each of the CH3X species, and their relative intensity and ΔA/A values vary systematically with halogen mass. Similar results are found in the CH3 deformation region. MVCD of the N–H stretches in NH3 is also presented for sake of comparison.

Rotationally resolved magnetic vibrational circular dichroism

“…With regard to the experimental NH 3 RR MVCD, there are significant differences between g~im and g~XV as shown in tables 3 and 4. The experimental g values found are (3,1) 1013.18 -7-5011 0"689 0.625 aR (3,2) 1012'44 -8"9039 0-817 0.620 aR (3,3) 1011"20 -7-7790 0-714 0.612 *aR (4,0) 1034.25 -6-3533 0"583 0'639 *aR (4,1) 1034.02 -6"8875 0-632 0.638 aR (4,2) 1033.32 -6"5698 0-603 0.634 aR (4,3) 1032.13 -7"3870 0.678 0.627 aR (4,4) 1030-42 -6.2780 0'576 0'618 aR (5,1) 1054.92 -5.4674 0'502 0'650 aR (5,2) 1054"26 -6"3600 0.584 0'647 aR (5,3) 1053-13 -8.7589 0.804 0.641 aR (5,4) 1051'51 -8'5563 0.786 0-634 aR (5,5) 1049.34 -8.3849 0.770 0-624 *aR (6,0) 1076-04 -7.6953 0"706 0-663 *aR (6,1) 1075.83 -10.0790 0'925 0.662 aR (6,2) 1075.21 -7'9050 0.726 0-660 aR (6,3) 1074. 16 -8.1976 0"753 0-655 aR (6,4) 1072"63 -5'5702 0.511 0-648 aR (6,5) 1070-60 -7-1968 0"661 0"640 aR (6,6) 1067-98 -5-7535 0"528 0"630 a aR(J",K') is a commonly used notation for the rotation-vibration double-inversion transitions where a and s indicate the parity of the total eigenfunction of the ground state with respect to the inversion of all particle about the centre of mass.…”

“…B terms (due to coupling between states as caused by the magnetic field) have only minor contributions to the MVCD spectra seen with rotational resolution. In the condensed phase where rotations are quenched and, consequently, A term contributions are generally reduced, this is not always the case [1][2][3][4]. C term contributions, which arise from differences in the populations of the ground state components which are split in energy by the magnetic field, are also very small in this case [14,26].…”

“…Since then MVCD has been the focus of both experimental and theoretical studies in our laboratory. MVCD of several relatively small symmetric and spherical top molecules and a large number of porphyrin derivatives have been measured in the condensed phase [2][3][4][5][6][7][8][9][10]. Meanwhile, a theoretical model based on the vibronic coupling between ground and excited electronic states [11][12][13] has been proposed and used for the analysis of the MVCD of several of these molecules.…”

Observation and calculation of vibrational circular birefringence: A new form of vibrational optical activity

“…The achievement of a simple route to the routine measurement of VCB, as well as an equally simple route to the routine calculation of VCB, opens a new area of natural optical activity in general, and vibrational optical activity in particular. Also, included in this advance is the potential to measure magnetic vibrational optical activity as both magnetic VCD (MVCD), as previously studied extensively for small non-chiral molecules, 34,35 and now MVCB. Important applications of direct measurements of VCB are foreseen to arise for complex chiral materials for which theoretical simulation is beyond the scope of current theoretical calculations, an important example of which is the search for chiral negative index materials.…”

Recent Trends in Chiroptical Spectroscopy: Theory and Applications of Vibrational Circular Dichroism and Raman Optical Activity

“…MVCD was firstly reported in 1981 for CH 3 I and (CH 3 ) 4 Si . On a dedicated instrument, other measurements of many small symmetric molecules followed soon, such as of methyl halides, CH 3 OH and CH 3 SH . Using the same “MVCD” instrument, low‐lying electronic transitions could be measured as well, such as in organometallic compounds .…”