The structures of neutral HCCX are well-established experimentally (Table ). The CCSD/cc-pVTZ optimized geometries are in very good agreement with experiment and recent high-level calculations. The shortening of the CC bond in HCCF compared to the other HCCX is a result of reduced interactions between the CC triple bond and the very electronegative fluorine. HCCCl, HCCBr, HCCI The highest occupied molecular orbitals of HCCX are shown in Figure and the geometries of the ground state of the haloacetylene cations (X states) are listed in Table . The X states of the HCCX cations are generated by removing an electron from the * orbital, which has a node between the CC  orbital and the -type lone pair on X. As a result, the CX bond is shorter than in the neutral while the CC bond is longer. The calculated bond lengths and their values as a function of the halogen are in very good agreement with the estimated experimental structures and trends. The changes in bond length on ionization have been approximated by Heilbronner based on the changes in spacing and intensities of the fine structure in photoelectron spectra. These differences are added to the bond length in Table to provide semiquantitative estimates of the geometries for the X and A states of the cations. Maier and coworkers measured the rotational constants for the X and A states of HCCX and DCCX cations (X = Cl, I) and used the R(CH) and R(CC) distances estimated by Heilbronner to obtain R(CX) bond lengths for the X and A states of HCCX. The very good agreement between the CCSD and EOMCCSD-IP optimized geometries of the X states indicates that EOMCCSD-IP should be very suitable for the excited A states of the cations. The first excited states of HCCX + (A states) are generated by removing an electron from the second highest occupied orbital, a  orbital with no node between the CC  orbital and the -type lone pair on the halogen (see Figure ). Because the A states of the HCCX cations have the same electronic symmetry as the X states, CCSD calculations would collapse to the lower energy X state. To avoid this problem, the A states have been calculated with the EOMCCSD-IP method, which involves ionization and excitation of the CCSD wavefunction of the neutral molecule. Since the A state is formed by removing an electron from a  bonding orbital, both the CC and CX bonds are longer than in the neutral HCCX. HCCF, however, is a special case. The optimized linear geometry of HCCF has a pair of imaginary frequencies (see below). Further optimization yields a trans-bent geometry. In contrast to the other haloacetylenes, the fluorine lone pair orbitals in HCCF are significantly lower in energy than the CC  orbital. As can be seen in Figure , the HCCF  orbital (-20.66 eV) is dominated by the fluorine lone pair. Furthermore, it is comparable in energy to the highest  bonding orbital (-19.68 eV). Ionizing from the  bonding orbital yields a configuration that is slightly lower in energy than the configuration formed by ionizing from the highest  orbital. Bending the CCF and CCH angles to allow these two configurations to interact thereby stabilizing the bent structure for the A state. (1.066) (1.067) (1.062) a present calculations with the cc-pVTZ basis set, b the linear structure for the A state of HCCF + has 2 imaginary frequencies; the trans bent structure is a minimum: R(CF) = 1.502 Å, R(CC) = 1.217 Å, R(CH) = 1.074 Å, CCF = 145.9, CCH = 162.0; c the numbers in parenthesis are geometry differences obtained Heilbronner added to the geometries of the neutral HCCX in Table , d ref , e ref The difference in geometry between X and A states is a key factor in the rate of decay of a coherent superposition of the X and A states. This has been studied in detail for HCCI. The largest changes are in the R(CX) bond lengths (0.194 Å, 0.178 Å and 0.162 Å for HCCCl, HCCBr and HCCI, respectively). The greater calculated change in R(CCl) compared to R(CI) suggests that decoherence could be somewhat faster for HCCCl than for HCCI.