Rebecca K. Milburn

A study of complexes Mg(NH3)n+· and Ag(NH3)n+, where n = 1–8: competition between direct coordination and solvation through hydrogen bonding

Abstract Density functional calculations at B3LYP/6-31+G(d) and B3LYP/DZVP are reported for Mg(NH 3 ) n +· , where n = 1–6 and for some solvated ions Mg(NH 3 ) n +· … NH 3 ( n = 1–3, 6). After correction for basis set superposition errors, the enthalpies for sequential addition of NH 3 to Mg +· resulting from direct coordination to the metal are 38.1, 26.6, 21.2, 13.7, 12.1, and 11.3 kcal mol −1 . The free energies for these same addition reactions are all negative, although for complexes with n ≥ 4 the values are very small. Attempts at optimising structures with higher coordination numbers all resulted in the formation of solvated octahedral complexes. Enthalpies for solvation through hydrogen bonding to one of the ligated NH 3 molecules are all less than 16 kcal mol −1 and decrease rapidly as the number of ligated NH 3 molecules increases. Molecular orbital calculations at B3LYP/DZVP have been used to optimise structures for ions Ag(NH 3 ) n + , where n = 1–6. The five-coordinate and six-coordinate structures have very small binding enthalpies (4.3 and 2.6 kcal mol −1 ) and the free energies for formation of these ions are positive. The binding energies for the addition of the first and second NH 3 molecules added to Ag + are 40.1 and 36.1 kcal mol −1 , while those for the third and fourth additions are much smaller (15.1 and 11.0 kcal mol −1 ). Adducts up to n = 3 have been detected in electrospray experiments. The first three adducts of Ag + with NH 3 have been formed in the selected ion flow tube apparatus and multicollision induced dissociation experiments show Ag(NH 3 ) 3 + to have a lower binding enthalpy than both Ag(NH 3 ) 2 + and Ag(NH 3 ) + .

Magnesium chemistry in the gas phase: calculated thermodynamic properties and experimental ion chemistry in H2–O2–N2 flames

Molecular orbital calculations were carried out for the neutral and ionic species that occur in the gas-phase chemistry of magnesium in the presence of oxygen and hydrogen including Mg 1 , MgO, MgOH, MgOH 1 , MgOH2 , Mg(OH)2, HOMgOH2 , and the hydrate structure MgOH 1 ...( OH 2). Standard enthalpies of formation for these species were obtained from single-point calculations at the QCISD(T)(full)/6-31111G(2df,p) and CCSD(T)(full)/6-31111G(2df,p) levels of theory using geometrical parameters obtained from MP2(full)/6-31111G(d,p) optimizations. These DHf values provide a recommended and self-consistent set with uncertainties as small as 612.6 kJ mol 21 (63 kcal mol 21 ) for deriving thermodynamic properties. The properties of interest include the proton affinities PA298 of MgO, MgOH, and Mg(OH)2, ionization energy IE0 of MgOH, bond dissociation energies D0 of Mg‐O, Mg‐OH, MgO‐H, HOMg‐OH, Mg‐OH 1 , H2O‐Mg 1 , and H2O‐MgOH 1 ; the latter two are hydration energies. Values in the literature, both experimental and theoretical, for many of these quantities show considerable scatter and a detailed comparison is made. Magnesium ions in fuel-rich, H2‐O2‐N2 flames at atmospheric pressure in the temperature range 1820 ‐2400 K were investigated experimentally by sampling the flames doped with magnesium through a nozzle into a mass spectrometer. It was shown that the interconversion of Mg 1 and MgOH 1 proceeds by way of the three-body reaction of Mg 1 with OH, in support of Sugden’s criterion that this occurs in flames if the weak HO‐Mg 1 bond has a dissociation energy less than 335 kJ mol 21 . The proton affinity PA 298 (MgO) was measured to be 1056 6 29 kJ mol 21 (252 6 7 kcal mol 21 ) along with approximate values for PA298 (MgOH) 5 919 kJ mol 21 (220 kcal mol 21 ) and PA298 [Mg(OH)2] 5 878 kJ mol 21 (210 kcal mol 21 ). Values were estimated for the electron‐ion recombination coefficient for MgH mOn molecular ions of 2 3 10 27 cm 3 molecule 21 s 21 , and for Mg 1 of 4 3 10 224 T 21 cm 6 molecule 22 s 21 similar to those for alkali metal ions. Finally, small rate coefficients for the chemi-ionization of magnesium via the reactions of Mg 1 OH and MgO 1 H to give MgOH 1 were estimated to be 4.832 3 10 29 exp(255 700/T) and 3.341 3 10 29 exp(232 970/T )c m 3 molecule 21 s 21 , respectively. (Int J Mass Spectrom 184 (1999) 153‐173)

Experimental and theoretical studies of the basicity and proton affinity of SiF4 and the structure of SiF4H

A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF4 and the structure of SiF4H+. The kinetics and energetics for the transfer of a proton between SiF4, N2, and Xe have been explored experimentally in helium at 0.35±0.02 torr and 297±3 K with a selected-ion flow tube apparatus. The results of equilibrium constant measurements are reported that provide a basicity and proton affinity for SiF4 at 297±3 K of 111.4±1.0 and 117.7±1.2 kcal mol−1, respectively. These values are more than 2.5 kcal mol−1 lower than currently recommended values. The basicity order was determined to be GB(Xe)>GB(SiF4)>GB(N2), while the proton-affinity order was shown to be PA(Xe)>PA(N2)>PA (SiF4). Ab initio molecular orbital computations at MP4SDTQ(fc)/6-311++G(3df,3pd) using geometries from B3LYP/6-31+G(d,p) indicate a value for PA(SiF4)=118.7 kcal mol−1 that is in good agreement with experiment. Also, the most stable structure of SiF4H+ is shown to correspond to a core SiF3+ cation solvated by HF with a binding energy of 43. 9 kcal mol−1. Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF.

Small-ring carbenes carrying a positive charge: the effects of substituting by CN in c-C4H3+

Calculations at B3LYP/6-311G(d) show c-C4H3 to have a singlet ground state, with the triplet being 19.5 kcal mol 21 higher in energy. Singlet c-C4H3 has a puckered ring structure (Cs symmetry) with an out-of-plane angle of 51.4°. The barrier to inversion via the planar ring structure ( C2v symmetry) is 29.1 kcal mol 21 . In the puckered ring the cross‐ring distance between CH groups is 1.793 A and the Wiberg bond order is 0.436, indicating considerable interaction. The stability of the singlet state then is attributed to homoaromaticity. Ion c-C4H3 in its triplet state is planar, but with alternate bonds being long and short, reminiscent of the geometries of c-C4H4 and c-C4H4 . Substitution of H atoms by CN groups stabilises the triplet state relative to the singlet, and for C 4(CN)3 the triplet is only 8.4 kcal mol 21 higher. Substitution also reduces the out-of-plane angle of the ions in the singlet state, going from 51.4° in the unsubstituted ion c-C4H3 to 43.4 kcal mol 21 in C4(CN)3 .AC N group adjacent (a) to the carbene centre reduces the barrier by ;7 kcal mol 21 whereas substitution at the opposite carbon (b position) reduces it by ;2 kcal mol 21 . These effects are roughly additive, with C4(CN)3 having a barrier to inversion of only 12.9 kcal mol 21 . (Int J Mass Spectrom 195/196 (2000) 393‐399)

Astrochemistry of Magnesium Cations with Hydrogen Cyanide and Cyanoacetylene: Possible Formation of Cyclic Tetramers of Cyanoacetylene

Laboratory measurements with the SIFT technique have shown that magnesium radical cations in a helium bath gas at 0.35 Torr and 294 K are unreactive with hydrogen cyanide but initiate a chemical sequence in cyanoacetylene that leads to the formation of Mg(HC3N)n+• cluster ions with n up to 7. Rate‐coefficient measurements for sequential addition of cyanoacetylene to Mg+• indicate an extraordinary pattern in alternating chemical reactivity while multiple‐collision induced dissociation experiments revealed an extraordinary stability for the Mg(HC3N)4+• cluster radical cation. Molecular orbital calculations with density functional theory (DFT) have provided structures and energies for the observed Mg(HC3N)1–4+• cations. These calculations indicate that the path of formation of Mg(HC3N)4+• appears to involve ligand‐ligand interactions mediated by Mg+• to form 2,4,6,8‐tetracyanosemibullvalene‐Mg+• or 1,2,5,6‐tetracyano‐1,3,5,7‐cyclooctatetraene‐Mg+• cations. A case is made for the formation of similar complex ...