Nonempirical LCAO MO SCF studies of the group IIa dihalides BeF2, MgF2, and CaF2

Abstract

Nonempirical LCAO MO SCF calculations are reported for the ground, C 2 " states of the Group IIa dihalides, BeF 2 , MgF2' and CaF 2 . These calculations demonstrate the importance of 3d orbitals in the bonding trends of the dihalides and, hence, in the determination of the equilibrium bond angles and the bending force constants. The calculations on BeF 2 indicate that d orbitals play an important role in the bonding but do not preferentially alter the general features of the total energy curve at any bond angle. In the case of MgF2' d orbitals are found to preferentially lower the energy of the nonlinear configurations. However, for both species, the ground-state equilibrium bond angle is predicted to be 180°. Configuration interaction studies also support the fact that, in the gas phase, the linear configuration is most stable. On the basis of s (1 s, 2s, 3s, 4s, 2p, and 3p orbitals on Ca) basis set calculations, it is predicted that CaF2 is linear. However, when 3d orbitals are added to the s basis set, the predicted equilibrium bond angle is 145°. This is in good agreement with the value of 141° deduced from matrix-isolation spectroscopy. The bonding in these molecules is discussed with reference to the nonempirical molecular correlation diagram.

Valence Angles in Group II Dihalide Molecules

“…This also leads to an increase in valence angle. It is not surprising, therefore, that in CaF2 the experimental angle is 141 0 [7], and that a recent Stark effect study of gaseous BaCh [14] leads to an angle < 135 0 • These values are quite in keeping with our general theory.…”

“…There is clearly no possible chance that any dihalide of these atoms should be nonlinear. Finally we may compare our general theory with the particular calculations of Gole et al [7]. These authors showed that in MgF2 and CaF2 the inclusion of d orbitals in the basis set tended to favor a bent conformation, and in CaF2 succeeded in making this more stable than the linear conformation.…”

“…It would be desirable, therefore, to provide some alternative discussion. One such purely numerical discussion has been provided by Gole, Siu and Hayes [7] who performed an ab initio LCAO-MO-SCF study of BeF2, MgF2 and CaF2. This confirmed the experimental observation that the first two molecules are linear.…”

Structure and vibrational frequencies of CaF2 aggregates isolated in a low temperature matrix

“…This molecule is an example of a non linear dihalide. Theoretical investigations have also been reported in the literature [4].…”

The vibration-rotation emission spectrum of hot BeF2

“…The failure of the VSEPR model and Walsh's rules to predict these bent structures prompted the development of improved models. Two physical models, d-orbital participation [22][23][24][25][26][27][28][29][30][31] and core polarization, 11,12,[32][33][34][35][36][37][38][39][40][41][42][43][44][45] were used to explain these unexpected bent structures and have been discussed in detail in the reviews by Hargittai 2 and Kaupp. 46 In the d-orbital participation model, the lighter metals ͑e.g., Be and Mg͒ have only valence ns and np orbitals for covalent bonding leading to sp hybridization, which results in a linear structure.…”