Unconventional CH(δ+)···N hydrogen bonding interactions in the stepwise solvation of the naphthalene radical cation by hydrogen cyanide and acetonitrile molecules.

Equilibrium thermochemical measurements using the mass-selected ion mobility (MSIM) technique have been utilized to investigate the binding energies and entropy changes of the stepwise association of hydrogen cyanide (HCN) and acetonitrile (CH3CN) molecules with the naphthalene radical cation (C10H8˙(+)) in the gas phase forming the C10H8˙(+)(HCN)n and C10H8˙(+)(CH3CN)n clusters with n = 1-3 and 1-5, respectively. The lowest energy structures of the C10H8˙(+)(HCN)n and C10H8˙(+)(CH3CN)n clusters for n = 1-2 have been calculated using the M062X and ω97XD methods within the 6-311+G** basis set, and for n = 1-6 using the B3LYP method within the 6-311++G** basis set. In both systems, the initial interaction occurs through unconventional CH(δ+)···N ionic hydrogen bonds between the hydrogen atoms of the naphthalene cation and the lone pair of electrons on the N atom of the HCN or the CH3CN molecule. The binding energy of CH3CN to the naphthalene cation (11 kcal mol(-1)) is larger than that of HCN (7 kcal mol(-1)) due to a stronger ion-dipole interaction resulting from the large dipole moment of CH3CN (3.9 D). On the other hand, HCN can form both unconventional hydrogen bonds with the hydrogen atoms of the naphthalene cation (CH(δ+)···NCH), and conventional linear hydrogen bonding chains involving HCN···HCN interactions among the associated HCN molecules. HCN molecules tend to form "externally solvated" structures with the naphthalene cation where the naphthalene ion is hydrogen bonded to the exterior of an HCN···HCN chain. For the C10H8˙(+)(CH3CN)n clusters, "internally solvated" structures are favored where the acetonitrile molecules are directly interacting with the naphthalene cation through CH(δ+)···N unconventional ionic hydrogen bonds. In both the C10H8˙(+)(HCN)n and C10H8˙(+)(CH3CN)n clusters, the sequential binding energy decreases stepwise to about 6-7 kcal mol(-1) by three HCN or CH3CN molecules, approaching the macroscopic enthalpy of vaporization of liquid HCN (6.0 kcal mol(-1)).

Communication: Ion mobility of the radical cation dimers: (Naphthalene)2(+•) and naphthalene(+•)-benzene: Evidence for stacked sandwich and T-shape structures.

Dimer radical cations of aromatic and polycyclic aromatic molecules are good model systems for a fundamental understanding of photoconductivity and ferromagnetism in organic materials which depend on the degree of charge delocalization. The structures of the dimer radical cations are difficult to determine theoretically since the potential energy surface is often very flat with multiple shallow minima representing two major classes of isomers adopting the stacked parallel or the T-shape structure. We present experimental results, based on mass-selected ion mobility measurements, on the gas phase structures of the naphthalene(+⋅) ⋅ naphthalene homodimer and the naphthalene(+⋅) ⋅ benzene heterodimer radical cations at different temperatures. Ion mobility studies reveal a persistence of the stacked parallel structure of the naphthalene(+⋅) ⋅ naphthalene homodimer in the temperature range 230-300 K. On the other hand, the results reveal that the naphthalene(+⋅) ⋅ benzene heterodimer is able to exhibit both the stacked parallel and T-shape structural isomers depending on the experimental conditions. Exploitation of the unique structural motifs among charged homo- and heteroaromatic-aromatic interactions may lead to new opportunities for molecular design and recognition involving charged aromatic systems.

Proton-bound dimers of nitrogen heterocyclic molecules: substituent effects on the structures and binding energies of homodimers of diazine, triazine, and fluoropyridine.

The bonding energies of proton-bound homodimers BH(+)B were measured by ion mobility equilibrium studies and calculated at the DFT B3LYP/6-311++G** level, for a series of nitrogen heterocyclic molecules (B) with electron-withdrawing in-ring N and on-ring F substituents. The binding energies (ΔH°(dissoc)) of the proton-bound dimers (BH(+)B) vary significantly, from 29.7 to 18.1 kcal/mol, decreasing linearly with decreasing the proton affinity of the monomer (B). This trend differs significantly from the constant binding energies of most homodimers of other organic nitrogen and oxygen bases. The experimentally measured ΔH°(dissoc) for (1,3-diazine)2H(+), i.e., (pyrimidine)2H(+) and (3-F-pyridine)2H(+) are 22.7 and 23.0 kcal/mol, respectively. The measured ΔH°(dissoc) for the pyrimidine(·+)(3-F-pyridine) radical cation dimer (19.2 kcal/mol) is signifcantly lower than that of the proton-bound homodimers of pyrimidine and 3-F-pyridine, reflecting the stronger interaction in the ionic H-bond of the protonated dimers. The calculated binding energies for (1,2-diazine)2H(+), (pyridine)2H(+), (2-F-pyridine)2H(+), (3-F-pyridine)2H(+), (2,6-di-F-pyridine)2H(+), (4-F-pyridine)2H(+), (1,3-diazine)2H(+), (1,4-diazine)2H(+), (1,3,5-triazine)2H(+), and (pentafluoropyridine)2H(+) are 29.7, 24.9, 24.8, 23.3, 23.2, 23.0, 22.4, 21.9, 19.3, and 18.1 kcal/mol, respectively. The electron-withdrawing substituents form internal dipoles whose electrostatic interactions contribute to both the decreased proton affinities of (B) and the decreased binding energies of the protonated dimers BH(+)B. The bonding energies also vary with rotation about the hydrogen bond, and they decrease in rotamers where the internal dipoles of the components are aligned efficiently for inter-ring repulsion. For compounds substituted at the 3 or 4 (meta or para) positions, the lowest energy rotamers are T-shaped with the planes of the two rings rotated by 90° about the hydrogen bond, while the planar rotamers are weakened by repulsion between the ortho hydrogen atoms of the two rings. Conversely, in ortho-substituted (1,2-diazine)2H(+) and (2-F-pyridine)2H(+), attractive interactions between the ortho (C-H) hydrogen atoms of one ring and the electronegative ortho atoms (N or F) of the other ring are stabilizing, and increase the protonated dimer binding energies by up to 4 kcal/mol. In all of the dimers, rotation about the hydrogen bond can involve a 2-4 kcal/mol barrier due to the relative energies of the rotamers.

Substituent effects on noncovalent bonds: complexes of ionized benzene derivatives with hydrogen cyanide.

Here, we report the first experimental and computational study of the noncovalent binding energies and structures of ionized benzenes containing electron-withdrawing substituents solvated by one to four HCN molecules. Measured by ion mobility mass spectrometric equilibrium studies, the bond dissociation enthalpies of the first HCN molecule to the fluorobenzene (C6H5F(•+)), 1,4-difluorobenzene (C6H4F2(•+)), and benzonitrile (C6H5CN(•+)) ions (11.2, 11.2, and 9.2 kcal/mol, respectively) are similar to those of HCN with the benzene (C6H6(•+)) and phenyacetylene (C6H5CCH(•+)) radical cations (9.2 and 10.5 kcal/mol, respectively). DFT calculations at the B3LYP/6-311++G(d,p) level show that HCN can form in-plane hydrogen bonds to ring hydrogens, or bind electrostatically to positively charged carbon centers in the ring. The electron-withdrawing substituents increase the bond energy by increasing the partial positive charge on the ring hydrogens that form CH(δ+)---NCH bonds, and by creating a π hole, as evidenced by positive charge centers on the fluorinated ring carbons for electrostatically bonded isomers. In the complexes of benzonitrile(•+), similar to benzene(•+), hydrogen bonded planar isomers have the lowest energy. In the complexes of (fluorinated benzene)(•+), the lowest energy isomers are electrostatically bonded where HCN is perpendicular to the ring and its dipole points to a positively charged ring carbon. However, in all cases the planar hydrogen-bonded and vertical electrostatic isomers have similar binding energies within 1 kcal/mol, although HCN interacts with different sites of the ionized benzenes in these isomers, suggesting that the observed cluster populations are mixtures of the planar and vertical isomers. Further HCN molecules can bind directly to unoccupied ring CH hydrogens or bind to the first-shell HCN molecules to form linear HCN---HCN--- hydrogen bonded chains. The binding energies decrease stepwise to about 6-7 kcal/mol by 4 HCN molecules, approaching the macroscopic enthalpy of vaporization of liquid HCN (6.0 kcal/mol).

Meteorite nanoparticles as models for interstellar grains: synthesis and preliminary characterisation.

Dust particles and their interaction with gases play important roles in star formation and in solar nebulae. Appropriate model dust grains are needed for the laboratory simulation of gas-grain interactions. Nanoparticles formed from carbonaceous meteorites may be particularly suitable, as these particles are formed from materials that were formed originally from interstellar/nebula dust. Extending our previous studies with grounded meteorite powders, we demonstrate here the production of nanoparticles formed from meteorites using the laser desorption/controlled condensation method developed in our laboratory. The product nanoparticle aggregates have porous, web-like morphologies similar to interstellar dust grains, indicating that they can present large specific surface areas for gas/grain interactions. In this paper, we present polarisation modulation reflection-absorption infrared spectra (PM-RAIRS) of supported thin films and compare these spectra with the known silicate bands in the spectra of interstellar dust recorded during the ISO mission. We also report an ultrahigh vacuum (UHV) temperature programmed desorption (TPD) study of the adsorption of CO on the supported nanoparticle films. The latter allow us to estimate the CO binding energy on the meteorite nanoparticles as 13.5 +/- 3.0 kJ mol(-1), cf. a value of 9.8 +/- 0.2 kJ mol(-1) for CO binding to a water ice substrate. Such thermochemical data can be useful for computational modelling of gas-grain interactions under the diverse conditions in interstellar clouds and solar nebulae.

A critical period for the impact of vestibular sensation on ferret motor development.

Children with hearing deficits and hypofunctioning vestibular receptors frequently have delayed motor development. This study focuses on when the vestibular system needs to be active for normal motor behavior development and the maturation of the soleus muscle in the ferret. Both vestibular labyrinths were removed from ferrets at Postnatal day 10 (P10), P21, or P45 and the resulting data were compared with ferrets that had undergone a sham surgery at the same ages. The animals were sacrificed at P120 (young adult ferret). The resulting data from these ferrets revealed that standing and walking balance was significantly affected when the vestibular system was eliminated at or before P21. The soleus of P10 and P21 animals generally had smaller diameter muscle fibers and proportionally less type I Myosin Heavy Chain (MHC) and more type IIX MHC. The twitch contraction time of the soleus of the P21 group was significantly slower than the other groups. It appears that the vestibular system is important to motor and muscle fiber development in the ferret during the period before P21. The eyes are still closed at that age and all of the vestibular receptors are not fully mature. These findings imply a "critical period" for vestibular sensation and the development of a muscle that is important to standing balance.

Chemistry of styrene (water)n clusters, n = 1-5: spectroscopy and structure of the neutral clusters, deprotonation of styrene dimer cation, and implication to the inhibition of cationic polymerization.

The styrene-water binary clusters SW(n), with n = 1-5 have been studied by the (one-color) resonant two-photon ionization technique using the resonance of styrene. The structures and energetics of the neutral clusters are investigated using a search technique that employs Monte Carlo procedure. The strong tendency for water molecules to form cyclic hydrogen-bonded structures is clearly observed in the SW(n) structures starting from n =3. The results indicate that the spectral shifts correlate with the interaction energies between styrene and the water subcluster (W(n)) within the SW(n) clusters. Evidence is presented that points to (1) the formation of a covalent bonded styrene radical cation dimer following the 193 nm MPI of styrene neutral clusters, (2) proton transfer from the styrene dimer cation to the water or methanol subcluster, resulting in the formation of protonated water or methanol clusters and a styrene dimer radical, and (3) extensive solvation of the styrene dimer radical within the protonated solvent molecules. The proton-transfer reactions may explain the strong inhibition effects exerted by small concentrations of water or methanol on the cationic polymerization of styrene. These results provide a molecular level view of the inhibition mechanism exerted by protic solvents on the cationic polymerization of styrene.

Effects of electrode penetrations into the abducens nucleus of the monkey: eye movement recordings and histopathological evaluation of the nuclei and lateral rectus muscles.

Two adult rhesus monkeys that had undergone 2 years of electrode penetrations into their abducens and vestibular nuclei, for chronic eye movement studies, were examined histologically. An analysis of their VIth nucleus neurons and lateral rectus muscles revealed the following. Twenty-two percent of the large neurons (approximately 30 microm in diameter), on average, were missing and extensive neuropil disruption and gliosis was evident in the experimental side abducens nuclei as compared with the control side in each animal. While the lateral rectus muscles showed small, but inconsistent, changes in total fiber number, the muscle fiber diameters were altered, leading to a more homogenous muscle and making the typical orbital and global subdivisions of the muscle less distinct. Eye movement records from before and after the electrophysiological studies were comparable. We discuss how the complex architecture of the extraocular muscles as well as the possibility of polyneuronal innervation of single muscle fibers could explain our results.

Motoneurons of the lateral and medial rectus extraocular muscles in squirrel monkey and cat.

The goal of this study was to examine and compare the number and size of motoneurons in the cat and squirrel monkey abducens nucleus. We also examined medial rectus muscle motoneuron compartmentalization in the squirrel monkey oculomotor nucleus and compared those cells to abducens nucleus motoneurons. Retrograde labeling of the motoneurons, using cholera toxin conjugate of horseradish peroxidase (CTHRP) injected into cat and monkey lateral or medial rectus muscles, was observed after 24 h. The CTHRP was histochemically localized with tetramethylbenzidine. The slide-mounted sections were analyzed using a computerized imaging system. Cat abducens nucleus motoneurons showed a wide range of cell sizes (26.0-66.0 microm, mean = 37.2 +/- 6.2 microm), four or more dendrites per cell and an average of 1,418 cells within a relatively loosely packed nucleus. Squirrel monkey abducens nucleus motoneurons were significantly smaller than those in the cat with a narrower range of cell sizes (20.0-44.0 microm, mean = 31.7 +/- 3.8 microm), four or more dendrites per cell and an average of 2,473 cells densely packed within the nucleus. Squirrel monkey medial rectus muscle motoneurons were organized into MRa, MRb and MRc subgroups. MRa motoneurons comprise the primary innervation for the medial rectus muscle and were similar in size to abducens nucleus motoneurons while the MRc subgroup cells were significantly smaller in size. Similar relationships among medial rectus motoneurons have been seen in rhesus monkeys. The relationship of these anatomical findings to previous physiological results regarding the generation of extraocular muscle force in the squirrel monkey is discussed.

Contractile properties of the tongue's genioglossus muscle and motor units in the rat.

The contractile characteristics of individual mammalian tongue muscles have rarely been investigated, in contrast to spinal cord-innervated and extraocular muscles. Therefore, whole muscle and motor unit contractile forces, plus muscle fiber types, were studied in the genioglossus, the major protrusor muscle, of the rat tongue. The muscle, exclusively composed of fast-contracting units, could be activated from rostroventral hypoglossal nucleus sites only. The following figures represent the means of the contractile measures. Whole muscle twitch tension was 7.02 g, contraction time was 14.22 ms, fusion frequency was 104 Hz, maximum tetanic tension was 37.22 g, and fatigue index was 0.72. Single motor unit twitch tension was 45. 9 mg, contraction time was 11.7 ms, fusion frequency was 94.8 Hz, maximum tetanic tension was 241.95 mg, and fatigue index was 0.68. The genioglossus muscle appeared qualitatively similar to the rat styloglossus muscle, one of the two major retractor muscles of the tongue. The delineation of motor unit contractile characteristics in tongue muscle is important in our understanding of the control of tongue movement.

Extraocular motor unit and whole-muscle responses in the lateral rectus muscle of the squirrel monkey.

Because primate studies provide data for the current experimental models of the human oculomotor system, we investigated the relationship of lateral rectus muscle motoneuron firing to muscle unit contractile characteristics in the squirrel monkey. Also examined was the correlation of whole-muscle contractile force with the degree of evoked eye displacement. A force transducer was used to record lateral rectus whole-muscle or muscle unit contraction in response to abducens whole-nerve stimulation or stimulation of single abducens motoneurons or axons. Horizontal eye displacement was recorded using a magnetic search coil. (1) Motor units could be categorized based on contraction speed (fusion frequency) and fatigue. (2) The kt value (change in motoneuronal firing necessary to increase motor unit force by 1.0 mg) of the units correlated with maximum tetanic tension. (3) There was some tendency for maximum tetanic tension of this unit population to separate into three groups. (4) At a constant frequency of 100 Hz, 95% of the motor units demonstrated significantly different force levels dependent on immediately previous stimulation history (hysteresis). (5) A mean force change of 0.32 gm/ degrees and a mean frequency change of 4.7 Hz/ degrees of eye displacement were observed in response to whole-nerve stimulation. These quantitative data provide the first contractile measures of primate extraocular motor units. Models of eye movement dynamics may need to consider the nonlinear transformations observed between stimulation rate and muscle tension as well as the probability that as few as two to three motor units can deviate the eye 1 degrees.

Summation of extraocular motor unit tensions in the lateral rectus muscle of the cat.

Contractile measures on 67 single muscle units in the cat lateral rectus muscle were made in response to motoneuron stimulation. Simultaneous activation of four to five additional units, using muscle nerve stimulation, allowed an examination of unit force summation. Linear force addition was found in 73% of the units, while 25% added only about half of their twitch force to the twitch force of the nerve-activated units. "Nonadditive" units had significantly weaker twitch tensions than the units which added linearly. Lengthening or shortening the whole muscle, from maximal isometric settings, reduced whole muscle twitch tension as well as muscle unit tension. Injury to the lateral rectus muscle did not significantly alter whole muscle tension. These findings suggest that the known serial and branching arrangement of these muscle fibers, as well as the complex interfiber matrix, may help explain the force reduction in some muscle units and the whole muscle's resistance to insult.

Lateral rectus whole muscle and motor unit contractile measures with the extraocular muscles intact.

Both extracellular and intracellular stimulation of single motoneurons were shown to be similarly effective and consistent in eliciting contractile responses in single lateral rectus muscle motor units. The whole muscle was activated by stimulating the sixth nerve in the brain stem. Both whole muscle and motor unit contractile characteristics, under isometric conditions, were found to remain consistent regardless of whether this extraocular muscle was detached or left attached to the globe. In addition, whole muscle twitch and maximum tetanic tension evoked by sixth nerve stimulation was significantly less than would be predicted by the linear summation of individual motor unit twitch and maximum tetanic tensions.

Extraocular motoneuron stimulation frequency effects on motor unit tension in cat.

The contractile characteristics of 47 twitch and 3 nontwitch lateral rectus motor units in 11 cats were examined using two different stimulation paradigms derived from observed motoneuron firing frequencies in alert animals. The twitch units showed an average twitch tension (46.0 +/- 8.1 mg), contraction time (6.15 +/- 0.26 ms) and median fusion frequency (170 Hz) consistent with previous studies, kt value, defined as the slope of the relation between a series of constant frequency tetanic stimulation trains (lasting 200 ms) and tetanic tension, correlated with maximum tetanic tension (r = 0.984, p < 0.05). ktps value, defined as the slope of the relation between tetanic tension and a series of constant frequency stimulation trains (lasting 1975 ms) that immediately followed a 25-ms high-frequency burst (pulse/step paradigm), was similarly correlated (r = 0.853, p < 0.05) with maximum tetanic tension (x = 256.5 +/- 35.2 mg). ktps values were lower that kt values for each unit, but the units did not change their position in the numerical hierarchy. Eighty-four percent of the motor units produced different maximum tetanic tensions (x = 24 +/- 3%), when comparing pulse/step to constant frequency stimulation, but only 14% of those units produced a greater maximum tetanic tension during pulse/step stimulation. In contrast, 46% of the motor units contracted with more force during the step phase of the pulse/step paradigm than with constant frequency stimulation when the stimulation rate was below 120 Hz: 24% of the units contracted with less force. In addition, pulse/step stimulation frequency ranges above 120 Hz (step phase) were ineffective in increasing tension while higher frequencies continued to elicit tension increases during constant frequency stimulation. The impact of these tension variations on eye movement is discussed.

Lateral rectus EMG and contractile responses elicited by cat abducens motoneurons.

Stimulation of 41 single, abducens nucleus motoneurons in the cat evoked electromyographic (EMG) and contractile responses in the ipsilateral lateral rectus muscle. Separate, bipolar, fine wire EMG recording electrodes in the global and orbital muscle layers showed that 22 muscle units were confined to the global layer, 8 to the orbital layer, and 11 units were contained in both ("bilayer") muscle layers. "Bilayer" units demonstrated significantly greater twitch (P < or = 0.002) and maximum tetanic (P < or = 0.001) tensions as well as faster fusion frequencies (P < or = 0.022) than either global or orbital units. "Bilayer" units also showed the lowest average kt values (the slope of the linear relationship between motoneuron stimulation frequency and isometric tetanic tension). "Bilayer" units were predominantly fast fatigable (FF). Global units displayed all muscle unit types including all the nontwitch (NT) units. Orbital units were identified as slow fatigable (SF) and fast fatigue resistant (FR).

Relationship of the mechanical properties of the cat inferior oblique muscle to the anatomy of its motoneurons and nerve branches.

Physiologically, the contractile characteristics and electromyography (EMG) of cat inferior oblique (IO) muscle fibers supplied by the medial and lateral IO muscle nerve branches were studied by direct nerve stimulation. Anatomically, the brain stem locations and sizes of IO motoneuron soma were evaluated after retrograde labeling by horseradish peroxidase (HRP) through whole IO muscle nerves and/or through each medial or lateral IO muscle nerve branch. Stimulation of the lateral nerve branch elicited significantly (p < 0.005) slower twitch contraction times (8.0 +/- 1.5 ms) and lower fusion frequencies (217 +/- 46 Hz) than when the medial branch of the IO nerve was stimulated (average twitch contraction time = 6.8 +/- 1.1 ms; average fusion frequency = 260 +/- 34 Hz). The EMG wave shape responses in the global and orbital layers could be differentiated when the individual nerve branches were stimulated, but the response differences were not consistent among animals. The average diameter of IO motoneuron soma with axons in the lateral branch of the nerve were significantly smaller (p < 0.001) than the average diameter of those IO motoneuron soma associated with the medial branch of the nerve (27.9 +/- 7.2 vs. 32.9 +/- 7.2 microns). Regardless of which nerve branch was labeled, the full range of motoneuron soma sizes was found, and these were distributed throughout the IO subdivision of the oculomotor nucleus. These findings showed that muscle contraction time and motoneuron soma diameter were correlated with the IO nerve branch subjected to stimulation or exposed to HRP. But no topographic organization of motoneurons was found within the IO division of the oculomotor nucleus.

Extraocular motor units: type classification and motoneuron stimulation frequency-muscle unit force relationships.

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat. Using a combination of two motor unit properties, the fusion frequency and an index of fatigability, the population of twitch motor units could be separated into 4 subgroups: fast fatigable (FF), fast fatigue resistant (FR), slow fatigable (SF) and slow fatigue resistant (S). Nontwitch motor units, a fifth subgroup (NT), formed 10% of the total studied population. The twitch tension and the maximum tetanic tension of the FF motor unit type were significantly stronger than all other motor unit types. The use of frequency varying stimulation patterns did not further differentiate the motor unit types. The relation between a series of single motoneuron stimulation frequencies and the resultant single muscle unit forces generated a slope defined as a motor unit's kt value. Motor units with low kt values had higher twitch tensions, higher maximum tetanic tensions, higher fusion frequencies and lower fatigue indices than motor units with high kt values. Motoneuron recruitment was tested by electrical stimulation of the medial rectus subdivision of the contralateral oculomotor nucleus. No correlations were seen between recruitment order and the mechanical parameters of the single abducens motor units.