Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne

摘要

The singlet ground(X ~1SIGMA~+)and excited(~1SIGMA~-,~1DELTA)states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory.For the ground state,geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory,coupled cluster theory with single and double excitations(CCSD),CCSD with perturbative triple corrections [CCSD(T)],and CCSD with partial iterative triple excitations(CCSDT-3 and CC3).Physical properties computed for the global minimum(X ~1SIGMA~+ HCP)include harmonic vibrational frequencies with the cc-pV5Z CCSD(T)method of omega_1 = 3344 cm~(-1),omega_2=689 cm~(-1),and omega_3=1298 cm~(-1).Linear HPC,a stationary point of Hessian index 2,is predicted to lie 75.2 kcal mol~(-1)above the global minimum HCP.The dissociation energy D_0[HCP(X ~1SIGMA~+)->H(~2S)+ CP(X~2SIGMA~+)] of HCP is predicted to be 119.0 kcalmol~(-1),which is very close to the experimental lower limit of 119.1 kcalmol~(-1).Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods(EOM-CCSD,EOM-CCSDT-3,and EOM-CC3).Four stationary points were located on the lowest-lying excited state potential energy surface,~1SIGMA~-->~1A",with excitation energies T_e of 101.4 kcalmol~(-1)(~1A"HCP),104.6 kcalmol~(-1)(~1SIGMA~-HCP),122.3 kcal mol~(-1)(~1A"HPC),and 171.6 kcal mol~(-1)(~1SIGMA~-HPC)at the cc-pVQZ EOM-CCSDT-3 level of theory.The physical properties of the ~1A"state with a predicted bond angle of 129.5 deg compare well with the experimentally reported first singlet state(A ~1A").The excitation energy predicted for this excitation is T_0=99.4 kcal mol~(-1)(34 800 cm~(-1),4.31 eV),in essentially perfect agreement with the experimental value of T_0=99.3 kcal mol~(-1)(34 746 cm~(-1),4.308 eV).For the second lowest-lying excited singlet surface,~1DELTA->~1A',four stationary points were found with T_e values of 111.2 kcal mol~(-1)(2~1A' HCP),112.4 kcal mol~(-1)(~1DELTA HPC),125.6 kcal mol~(-1)(2~1A' HCP),and 177.8 kcal mol~(-1)(~1DELTA HPC).The predicted CP bond length and frequencies of the 2 ~1A' state with a bond angle of 89.8 deg(1.707 A,666 and 979 cm~(-1))compare reasonably well with those for the experimentally reported C ~1A' state(1.69 A,615 and 969 cm~(-1)).However,the excitation energy and bond angle do not agree well:theoretical values of 108.7 kcal mol~(-1)and 89.8 deg versus experimental values of 115.1 kcalmol~(-1)and 113 deg.