Metal-silicon cluster cations of the form MSi(n)+ (M = Cu, Ag, Cr) are produced in a molecular beam with pulsed laser vaporization. These species are mass-selected in a reflectron time-of-flight spectrometer and studied with laser photodissociation at 532 and 355 nm. For the noble metals copper and silver, photodissociation of the n = 7 and 10 clusters proceeds primarily by the loss of metal atoms, indicating that the metal is not located within the interior of silicon cages, and that metal-silicon bonding is weaker than silicon-silicon bonding. Chromium-silicon clusters for n = 7 also lose primarily the metal atom, but at n = 15 and 16 these dissociate via the loss of silicon, producing smaller metal-silicon species. This behavior is consistent with stronger metal-silicon bonding and encapsulated metal structures, as suggested previously by theory. MSi6(+) cations are produced efficiently in all of these photodissociation processes, indicating that these species have enhanced stability compared to other small clusters. Improved values are obtained for the ionization potentials of Si7 and Si10.

Ni+ (benzene)n (n = 1-6) and Ni+ (benzene)n Ar(1,2) (n = 1,2) are produced by laser vaporization in a pulsed nozzle cluster source. The clusters are mass selected and studied by infrared laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer. The excitation laser is an OPO/OPA system that produces tunable IR in the C-H stretching region of benzene. Photodissociation of Ni+ (benzene)n complexes occurs by the elimination of intact neutral benzene molecules, while Ni+ (benzene)n Ar(1,2) complexes lose Ar. This process is enhanced on resonances, and the vibrational spectrum is obtained by monitoring the fragment yield versus the infrared wavelength. Vibrational bands in the 2700-3300 cm(-1) region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. A dramatic change in the IR spectrum is seen at n = 3 and is attributed to the presence of external benzene molecules acting as solvent molecules in the cluster. The results of previous theoretical calculations are employed to investigate the structures, energetics, and vibrational frequencies of these complexes. The mono-benzene complex is found to have a C2v structure, with benzene distorted by the metal pi-bonding. The di-benzene complex is found to have a D2h structure, with both benzenes distorted. The comparison between experiment and theory provides intriguing new insight into the bonding in these prototypical pi-bonded organometallic complexes.

V+(N2)n clusters are generated in a pulsed nozzle laser vaporization source. Clusters in the size range of n = 3-7 are mass selected and investigated via infrared photodissociation spectroscopy in the N-N stretch region. The IR forbidden N-N stretch of free nitrogen becomes strongly IR active when the molecule is bound to the metal ion. Photodissociation proceeds through the elimination of intact N2 molecules for all cluster sizes, and the fragmentation patterns reveal the coordination number of V+ to be six. The dissociation process is enhanced on vibrational resonances and the IR spectrum is obtained by monitoring the fragmentation yield as a function of wavelength. Vibrational bands are red-shifted with respect to the free nitrogen N-N stretch, in the same way seen for the C-O stretch in transition metal carbonyls. Comparisons between the measured IR spectra and the predictions of density functional theory provide new insight into the structure and bonding of these metal ion complexes.

M(+)(acetone) ion-molecule complexes (M = Mg, Al, Ca) are produced in a pulsed molecular beam by laser vaporization and studied with infrared photodissociation spectroscopy in the carbonyl stretch region. All of the spectra exhibit carbonyl stretches that are shifted significantly to lower frequencies than the free-molecule value, consistent with metal cation binding on the oxygen of the carbonyl. Density functional theory is employed to elucidate the shifts and patterns in these spectra. Doublet features are measured for the carbonyl region of Mg(+) and Ca(+) complexes, and these are assigned to Fermi resonances between the symmetric carbonyl stretch and the overtone of the symmetric carbon stretch. The carbonyl stretch red shift is greater for Al(+) than it is for the Mg(+) and Ca(+) complexes. This is attributed to the smaller size of the closed-shell Al(+), which enhances its ability to polarize the carbonyl electrons. Density functional theory correctly predicts the direction of the carbonyl stretch shift and the relative trend for the three metals.

Potentiation of opioid analgesia can be achieved by the addition of midazolam intrathecally. At our institution, analgesia following open abdominal surgery is provided by continuous infusion of analgesic solutions either intravenously, intrathecally (incorporating midazolam) or epidurally. We report the results of a study comparing outcomes with these three analgesic regimens following major open abdominal surgery. This was an unblinded prospective audit of pain service intervention rates, pain scores and other outcomes after intravenous, intrathecal and epidural analgesia after open abdominal surgery in patients over 60 years of age. Both elective and emergency cases were included over a nine-month period. Patients ventilated for 24 hours or more were excluded. The analgesic regimens were as follows: 1. Intravenous: patient controlled analgesia with morphine+ketamine infusion 0.1 to 0.2 mg/kg/h. 2. Intrathecal: (morphine 10 g/ml+midazolam 100 g/ml+bupivacaine 0.05%) commenced at 2 ml/h. 3. Epidural: bupivacaine 0.125%+fentanyl 2 g/ml at 6 to 14 ml/h. Co-analgesic administration was as per our usual practice but was not standardised. The median number of calls per patient to the pain service differed between the intravenous (1), intrathecal (1) and epidural (3) groups. The number of unintentional analgesic regimen terminations differed between the intravenous (1), intrathecal (1) and epidural (5) groups. Pain scores differed significantly between groups and were lowest in the intrathecal group at all time points. The findings indicate that the intrathecal group had both a low requirement for postoperative interventions/ resources and excellent analgesia. It appears to be a suitable alternative to the other techniques.