The inter molecular potential of NH+ 4-Ar II. Calculations and experimental measurements for the rotational structure of the v 3 band

The mid-infrared spectrum of the v ₃,(t ₂) transition of the NH⁺ ₄-Ar complex has been recorded at rotational resolution using photofragmentation spectroscopy. The spectrum is divided into perpendicular and parallel subbands corresponding to transitions between different hindered internal rotor states. The P and R branches of the strongest perpendicular subbands are rotationally resolved providing rotational and centrifugal distortion constants. The widths of individual rotational lines are limited by the laser bandwidth of 0.02 cm^?1, giving a lower limit of 250 ps for the lifetime of the excited states. Effective intermolecular separations for each internal rotor state are determined from its rotational constant, after correction for the contribution due to Coriolis coupling between the internal and total rotational angular momenta. The absolute energies, rotational and distortion constants for the first few intermolecular bending and stretching levels of the ground intramolecular vibrational state are determined in a numerical solution to the rotation-intermolecular vibration Hamiltonian, employing a three-dimensional ab initio intermolecular potential. The results are compared with the experimental constants in order to assess the accuracy of the calculated potential. The relative energy levels from this calculation are also compared with those from a two-dimensional representation of the potential energy surface (‘fixed-R’ model) in order to judge directly the influence of the radial dependence of the potential.     

The intermolecular potential of NH+ 4-Ar I. Calculations for the internal rotor structure of the v 3 band

The intermolecular potential energy surface of the electronic ground state of the ammonium-argon ionic dimer, NH⁺ ₄-Ar, is calculated by ab initio methods using different levels of theory (MP2, MP4) and basis sets (aug-cc-pVXZ, X = D/T/Q). The deformation of the ammonium ion in the complex is shown to be small and its geometry is therefore fixed in these calculations to the tetrahedral structure optimized for the bare ion. The global minimum of the potential corresponds to a proton-bound structure with C_3v symmetry (R_e ≈ 3.4 Å, D_e ≈ 950 cm^?1) and the barrier to internal rotation between the four equivalent minima is around 200 cm^?1. The three-dimensional potential is expanded in tetrahedral harmonics whose radially dependent coefficients, V_i (R), are compared for the considered levels of theory. The rotation-intermolecular vibration Hamiltonian is solved using a two-dimensional fixed-R representation of the calculated potentials, V_i , ≡ V_i (R ^eff), where the effective intermolecular separation, RR^eff, is determined from the experimental rotational constants of the complex. The accuracy of these parametrized potential energy surfaces is judged by their ability to reproduce the hindered rotor subband structure in the experimental v ₃(t ₂) infrared band of the complex. The simulations using the potentials calculated at the MP2/aug-cc-pVTZ or higher levels of theory reproduce the coarse structure of the experimental spectrum well. Further improvement could be achieved by least-squares fitting the potential parameters to the observed subband positions. The fitted V ₃ and V ₄ parameters remain in close agreement with those determined from the ab initio calculations but the anisotropy of the potential is significantly different from that in a previous least-squares fit of V ₃ alone.     

Local Compliance Effects on the Global Pressure-Volume Relationship in Models of Intracranial Pressure Dynamics

The experimentally-measured pressure-volume relationship for the human intracranial system is a nonlinear ‘S-shaped’ curve with two pressure plateaus, a point of inflection, and a vertical asymptote at high pressures where all capacity for volume compensation is lost. In lumped-parameter mathematical models of the intracranial system, local compliance parameters relate volume adjustments to dynamic changes in pressure differences between adjacent model subunits. This work explores the relationship between the forms used for local model compliances and the calculated global pressure-volume relationship. It is shown that the experimentally-measured global relationship can be recovered using physiologically motivated expressions for the local compliances at the interfaces between the venous-cerebrospinal fluid (CSF) subunits and arterial-CSF subunits in the model. Establishment of a consistent link between local model compliances and the physiological bulk pressure-volume relationship is essential if lumped-parameter models are to be capable of realistically predicting intracranial pressure dynamics.