A microwave spectroscopic and quantum chemical study of propa-1,2-dienyl selenocyanate (H2CCHSeCN) and cyclopropyl selenocyanate (C3H5SeCN)

Abstract

The microwave spectra of propa-1,2-dienyl selenocyanate, H(2)C==C==CHSeC[triple bond]N, and cyclopropyl selenocyanate, C(3)H(5)SeC[triple bond]N, are reported. The spectra of the ground and two vibrationally excited states of the (80)Se isotopologue and the spectrum of the ground state of the (78)Se isotopologue were assigned for one rotameric form of H(2)C==C[double bond, length as m-dash]CHSeC[triple bond]N. This conformer is characterized by a C-C-Se-C dihedral angle of 129(5) degrees from synperiplanar (0 degrees ) and is shown to be the global minimum of H(2)C[double bond, length as m-dash]C[double bond, length as m-dash]CHSeC[triple bond]N. The spectra of the ground and of three vibrationally excited states of the (80)Se isotopologue, as well as of the ground state of the (78)Se isotopologue of one rotamer of C(3)H(5)SeC[triple bond]N were assigned. This conformer has a H-C-Se-C dihedral angle of 80(4) degrees from synperiplanar and is at least 3 kJ mol(-1) more stable than any other form of the molecule. The microwave study has been augmented by quantum chemical calculations at the B3LYP/6-311+ +G(3df,3pd) and MP2/6-311+ +G(3df,3pd) levels of theory.

Microwave and Quantum Chemical Study of Propargyl Selenocyanate (HC≡CCH2SeC≡N)

“…While ap is found to have exact C s symmetry in both the MP2 and in the B3LYP calculations, the MP2 C6-C1-Se4-C9 dihedral angle was predicted to be 60.0°in the sc, about 10°l ess than that found in the B3LYP calculations (70.4°). The finding that MP2 and B3LYP calculations both predict the existence of a conformer with exact C s symmetry whereas deviations of 5-10°are found in the dihedral angles characterizing the second rotamer have also been seen for other selenocyanates such as HCdCHSeCN, 8 H 2 CdCdCHSeCN, 9 and C 3 H 5 SeCN. 9 The CtC bond distances are typically 1.5 pm longer in MP2 than in the B3LYP calculations.…”

“…Cyclopropyl selenocyanate represents yet another conformational problem. In this case, a H−C−Se−C synclinal (80(4)° from synperiplanar) conformer was found to be the preferred …”

“…8 Very recently, the MW spectra of allenyl (H 2 CdC)CHSeCN) and cyclopropyl selenocyanate (C 3 H 5 SeCN) were published. 9 One anticlinal conformer, which is the global minimum, was identified for H 2 CdCdCHSeCN. The CdC-Se-C dihedral angle is 129 (5)°i n this rotamer.…”

“…In this case, a H-C-Se-C synclinal (80(4)°f rom synperiplanar) conformer was found to be the preferred. 9 The conformational studies referred to above demonstrate that the selenocyanates have varied and interesting structural and conformational properties, which depend critically on the interaction between this group and neighboring functional groups. This first MW study of propargyl selenocyanate was undertaken because the interaction between the propargyl and the selenocyanate groups is different from the interactions in the selenocyanates already investigated by this technique [1][2][3]8,9 and discussed above.…”

“…The conformational studies referred to above demonstrate that the selenocyanates have varied and interesting structural and conformational properties, which depend critically on the interaction between this group and neighboring functional groups. This first MW study of propargyl selenocyanate was undertaken because the interaction between the propargyl and the selenocyanate groups is different from the interactions in the selenocyanates already investigated by this technique − ,, and discussed above. It should be mentioned that photoelectron spectra and quantum chemical calculations at the MP2/cc-pVTZ and B3LYP/cc-pVTZ levels of theory have already been reported for HCCCH 2 SeCN, as well as for several other allenyl and alkynyl selenols and selenocyanates …”