Metastable States of Dimethylammonium, (CH3)2NH2•

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Synthesis and properties of hypervalent electron-rich pentacoordinate nitrogen compounds

Isolation and structural characterization of hypervalent electron-rich pentacoordinate nitrogen species have not been achieved despite continuous attempts for over a century. Herein we report the first synthesis and isolation of air stable hypervalent electron-rich pentacoordinate nitrogen cationic radical (11-N-5) species from oxidation of their corresponding neutral (12-N-5) species. In the cationic radical species, the nitrogen centers adopt a trigonal bipyramidal geometry featuring a 3-center-5-electron hypervalent attractive interaction. The combination of single crystal X-ray diffraction analysis and computational studies revealed weak N-O interactions between the central nitrogen cation and oxygen atoms. This successful design strategy and isolation of air-stable pentacoordinate hypervalent nitrogen species allow further investigations on reactivity and properties resulting from these unusually weakly coordinating interactions in nitrogen compounds.

Long-Range N-N Bonding by Rydberg Electrons

By using high-level ab initio methods, we examine the nature of bonding between Rydberg electrons hosted by two four-coordinate nitrogen centers embedded in a hydrocarbon scaffold. The electronic structure of these species resembles that of diradicals, yet the diffuse nature of the orbitals hosting the unpaired electrons results in unusual features. The unpaired Rydberg electrons exhibit long-range bonding interactions, leading to stabilization of the singlet state (relative to the triplet) and a reduced number of effectively unpaired electrons. However, thermochemical gains due to through-space bonding are offset by strong Coulomb repulsion between positively charged nitrogen cores. The kinetic stability of these Rydberg diradicals may be controlled by a judicious choice of the molecular scaffold, suggesting possible strategies for their experimental characterization.

An electron-transfer photochromic crystalline MOF accompanying photoswitchable luminescence in a host–guest system

A new electron transfer type photoactive host–guest supramolecule was constructed by introducing (CH₃)₂NH₂⁺ cations to the MOF framework. The resulting compound 1 exhibits reversible photochromic property without using photochromic components.

Electron super-rich radicals. III. On the peculiar behavior of the aminodihydroxymethyl radical in the gas phase

In contrast to previously reported electron-super-rich trihydroxy-, triamino- and diaminohydroxymethyl radicals, the title aminodihydroxymethyl radical (1) generates a fraction of metastable species in the form of their deuterium isotopologues. The lifetimes of metastable radicals produced by femtosecond collisional electron transfer to aminodihydroxymethyl cations exceed 4 mus. The main fraction of 1 dissociates by fast loss of a hydroxyl hydrogen atom to form carbamic acid. Loss of an amino hydrogen atom is less facile and becomes <10% competitive at high internal energies or if the main dissociation is slowed down by deuterium isotope effects. RRKM calculations of unimolecular rate constants on a CCSD(T)/aug-cc-pVTZ potential energy surface gave a reasonably good fit for the competitive dissociations of 1 but not for the fraction of nondissociating radicals. The metastable species are attributed to excited electronic states which are predicted to have favorable Franck-Condon factors for being formed by collisional electron transfer.

Transition metals as electron traps. I. Structures, energetics, electron capture, and electron-transfer-induced dissociations of ternary copper-peptide complexes in the gas phase

Electron-induced dissociations of gas-phase ternary copper-2,2'-bipyridine complexes of Gly-Gly-Gly and Gly-Gly-Leu were studied on a time scale ranging from 130 ns to several milliseconds using a combination of charge-reversal ((+)CR(-)) and electron-capture-induced dissociation (ECID) measured on a beam instrument and electron capture dissociation (ECD) measured in a Penning trap. Charge-reduced intermediates were observed on the short time scale in the (+)CR(-) and ECID experiments but not in ECD. Ion dissociations following electron transfer or capture mostly occurred by competitive bpy or peptide ligand loss, whereas peptide backbone fragmentations were suppressed in the presence of the ligated metal ion. Extensive electron structure theory calculations using density functional theory and large basis sets provided optimized structures and energies for the precursor ions, charge-reduced intermediates, and dissociation products. The Cu complexes underwent substantial structure changes upon electron capture. Cu was calculated to be pentacoordinated in the most stable singly charged complexes of the [Cu(peptide-H)bpy](+*) type where it carried a approximately +1 atomic charge. Cu coordination in charge-reduced [Cu(peptide-H)bpy] intermediates depended on the spin state. The themodynamically more stable singlet states had tricoordinated Cu, whereas triplet states had a tetracoordinated Cu. Cu was tricoordinated in stable [Cu(peptide-H)bpy](-*) products of electron transfer. [Cu(peptide)bpy](2+*) complexes contained the peptide ligand in a zwitterionic form while Cu was tetracoordinated. Upon electron capture, Cu was tri- or tetracoordinated in the [Cu(peptide)bpy](+) charge-reduced analogs and the peptide ligands underwent prototropic isomerization to canonical forms. The role of excited singlet and triplet electronic states is assessed.

Gas-phase models for catalysis: alkane activation and olefin epoxidation by the triatomic cation Ag2O+

Electrospray ionization of aqueous silver nitrate is used for the preparation of the disilver-oxide cation Ag2O+ in the gas phase. The mass-selected cation is capable of activating C-H bonds of simple alkanes other than methane via H-atom abstraction, i.e., Ag2O+ + R-H --> Ag2OH+ + R* (R = C2H5, C3H7, C4H9). Clean O-atom transfer from Ag2O+ is observed with ethene as a neutral reagent, whereas oxygenation and allylic C-H abstraction compete in the case of propene. The gaseous Ag2O+ cation can thus be regarded as a minimalist model for the problems associated with the silver-mediated epoxidation of olefins more complex than ethene itself. The experimental findings are fully supported by the results of quantum chemical studies, thereby providing deep mechanistic insight into the reactions in the idealized gas phase, which also might have implications for further improvements in applied catalysis.

On the survival of peptide cations after electron capture: role of internal hydrogen bonding and microsolvation

Electron capture by both bare and microsolvated small peptide dications was investigated by colliding these ions with sodium vapor in an accelerator mass spectrometer to provide insight into processes that occur on the microsecond time frame. Survival of the intact peptide monocation after electron capture depends strongly on molecular size. For dipeptides, no intact reduced species were observed; the predominant ions correspond to loss of hydrogen and ammonia. In contrast, the intact reduced species was observed for larger peptides. Calculated structures indicate that the diprotonated dipeptide ions form largely extended structures with low probability of internal ionic hydrogen bonding (i.e., charge solvation) whereas internal ionic H-bonding occurs extensively for larger peptide dications. Solvation of the peptide ions with between one to seven methanol molecules reduces the total extent of H loss even for dipeptides where intact reduced species can survive more than a microsecond after electron capture. The yield of ions corresponding to cleavage of NCalpha bonds (c+ and z+* ions) does not depend strongly on peptide size but decreases with the extent of microsolvation for the dipeptide dications. H-bonding appears to play an important role for the survival of the intact reduced ions but less so for the formation of c+ and z+* ions. Our results indicate that electron capture predominantly occurs at the ammonium groups (at least 70 to 80%), and provides important new insights into the electron capture dissociation process.

Toward a general mechanism of electron capture dissociation

The effects of positive charge on the properties of ammonium and amide radicals were investigated by ab initio and density functional theory calculations with the goal of elucidating the energetics of electron capture dissociation (ECD) of multiply charged peptide ions. The electronic properties of the amide group in N-methylacetamide (NMA) are greatly affected by the presence of a remote charge in the form of a point charge, methylammonium, or guanidinium cations. The common effect of the remote charge is an increase of the electron affinity of the amide group, resulting in exothermic electron capture. The N-Calpha bond dissociation and transition state energies in charge-stabilized NMA anions are 20-50 kJ mol(-1) greater than in the hydrogen atom adduct. The zwitterions formed by electron capture have proton affinities that were calculated as 1030-1350 kJ mol(-1), and are sufficiently basic for the amide carbonyl to exothermically abstract a proton from the ammonium, guanidinium and imidazolium groups in protonated lysine, arginine, and histidine residues, respectively. A new mechanism is proposed for ECD of multiply charged peptide and protein cations in which the electron enters a charge-stabilized electronic state delocalized over the amide group, which is a superbase that abstracts a proton from a sterically proximate amino acid residue to form a labile aminoketyl radical that dissociates by N-Calpha bond cleavage. This mechanism explains the low selectivity of N-Calpha bond dissociations induced by electron capture, and is applicable to dissociations of peptide ions in which the charge carriers are metal ions or quaternary ammonium groups. The new amide superbase and the previously proposed mechanisms of ECD can be uniformly viewed as being triggered by intramolecular proton transfer in charge-reduced amide cation-radicals. In contrast, remote charge affects N-H bond dissociation in weakly bound ground electronic states of hypervalent ammonium radicals, as represented by methylammonium, CH3NH3*, but has a negligible effect on the N-H bond dissociation in the strongly bound excited electronic states. This refutes previous speculations that loss of "hot hydrogen" can occur from an excited state of an ammonium radical.

Confirmation of the “long-lived” tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations

Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.

Electron-rich radicals by neutralizationreionization mass spectrometry. Generation, dissociations and energetics of the hydrogen atom adduct to acetamide

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants.

A mass spectrometry study of tirapazamine and its metabolites. insights into the mechanism of metabolic transformations and the characterization of reaction intermediates

Tandem mass spectrometry methods were used to study the sites of protonation and for identification of 3-amino-1,2,4-benzotriazine 1,4-dioxide (1, tirapazamine), and its metabolites (3-amino-1,2,4-benzotriazine 1-oxide (3), 3-amino-1,2,4-benzotriazine 4-oxide (4), 3-amino-1,2,4-benzotriazine (5), and a related isomer 3-amino-1,2,4-benzotriazine 2-oxide (6). Fragmentation pathways of 3 and 5 indicated the 4-N-atom as the most likely site of protonation. Among the N-oxides studied, the 4-oxide (4) showed the highest degree of protonation at the oxygen atom. The differences in collision-induced dissociation of isomeric protonated 1-, 2- and 4-oxides allowed for their identification by LC/MS/MS. Gas phase and liquid phase protonation of tirapazamine occurred exclusively at the oxygen in the 4-position. A loss of OH radical from these ions (2(+)) resulted in ionized 3. Neutralization-reionization mass spectrometry (NR MS) experiments demonstrated the stability of the neutral analogue of protonated tirapazamine in the gas phase in the micro s time-frame. A significant portion of the neutral tirapazamine radicals (2) dissociated by loss of hydroxyl radical during the NR MS event, which indicates that previously proposed mechanisms for redox-activated DNA damage are reasonable. The activation energy for loss of hydroxyl radical from activated tirapazamine (2) was estimated to be approximately 14 kcal mol(-1). Stable neutral analogues of [3 + H](+) and [5 + H](+) ions were also generated in the course of NR MS experiments. Structures of these radicals were assigned to the molecules having an extra hydrogen atom at one of the ring N-atoms. Quantum chemical calculations of protonated 1, 3, 4 and 5 and the corresponding neutrals were performed to assist in the interpretation of experimental results and to help identify their structures.

Stereochemical interactions in ammonium dications, hypervalent diammonium cation-radicals and ammonium radicals. A B3-MP2 computational study

Hypervalent ammonium radicals and cation-radicals derived from cis and trans-cyclopentane-1,2-diamine have been studied computationally with the goal of elucidating intramolecular interactions between the ammonium and amine functional groups in a conformationally rigid model system. Hypervalent cis and trans-cyclopentane-1-ammonium-2-amine radicals are only marginally stable against dissociation by loss of an ammonium hydrogen atom and ammonia. The radicals are predicted to exhibit very similar dissociations originating from the ground and excited electronic states. The dissociation and transition state energies in bifunctional hypervalent ammonium radicals are analogous to those reported previously for monofunctional ammonium radicals. Stereochemical effects are predicted to occur in the formation and dissociations of hypervalent cis and trans-cyclopentane-1,2-diammonium cation-radicals. The cis-isomer is calculated to undergo very facile elimination of dihydrogen resulting from a dipolar H(+)....H(-) interaction between the ammonium groups. The trans-isomer is predicted to spontaneously dissociate to a complex of cyclopentene cation-radical and two ammonia molecules.

Generation and characterization of ionic and neutral P(OH)2+/* in the gas phase by tandem mass spectrometry and computational chemistry

The bicoordinated dihydroxyphosphenium ion P(OH)2+ (1+) was generated specifically by charge-exchange dissociative ionization of triethylphosphite and its connectivity was confirmed by collision induced dissociation and neutralization-reionization mass spectra. The major dissociation of 1+ forming PO+ ions at m/z 47 involved another isomer, O=P-OH2+ (2+), for which the optimized geometry showed a long P-OH2 bond. Dissociative 70-eV electron ionization of diethyl phosphite produced mostly 1+ together with a less stable isomer, HP(O)OH+ (3+). Ion 2+ is possibly co-formed with 1+ upon dissociative 70-eV electron ionization of methylphosphonic acid. Neutralization-reionization of 1+ confirmed that P(OH)2* (1) was a stable species. Dissociations of neutral 1, as identified by variable-time measurements, involved rate-determining isomerization to 2 followed by fast loss of water. A competitive loss of H occurs from long-lived excited states of 1 produced by vertical electron transfer. The A and B states undergo rate-determining internal conversion to vibrationally highly excited ground state that loses an H atom via two competing mechanisms. The first of these is the direct cleavage of one of the O-H bonds in 1. The other is an isomerization to 3 followed by cleavage of the P-H bond to form O=P-OH as a stable product. The relative, dissociation, and transition state energies for the ions and neutrals were studied by ab initio and density functional theory calculations up to the QCISD(T)/6-311+G(3df,2p) and CCSD(T)/aug-cc-pVTZ levels of theory. RRKM calculations were performed to investigate unimolecular dissociation kinetics of 1. Excited state geometries and energies were investigated by a combination of configuration interaction singles and time-dependent density functional theory calculations.

Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization-reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728-738 kJ mol-1 depending on the optimization method used to calculate the equilibrium geometry of the CH3O(H)-NO2+ ion (2+). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol-1 for the formation of the syn-syn, anti-syn, and syn-anti ion rotamers 3a+, 3b+, and 3c+, respectively. Ion 2+ was prepared by an ion-molecule reaction of NO2+ with methanol and used to generate the transient CH3O(H)-NO2. radical (2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a(+)-3c+ with 2+ that was used to generate transient radicals 3a-3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO2. Radicals 3a-3c were calculated to be weakly bound. When formed by vertical neutralization, 3a-3c dissociated completely on the 4.2 microseconds time scale of the experiment. The main dissociations of 3a-3c were formations of CH3O. + HONO and CH3ONO + OH.. The gas-phase chemistry of radicals 3a-3c and their dissociation products, as studied by neutralization-reionization mass spectrometry, was dominated by Franck-Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a-3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.

Franck-Condon Dominated Chemistry. Formation and Dissociations of the Dimethylhydroxysulfuranyl Radical

The structure and energetics of the hydroxyl radical adduct to dimethyl sulfide (DMS) was revisited using high level ab initio calculations. Density functional theory B3LYP/6-31++G(2d,p) and perturbational MP2(FULL)/6-31++G(2d,p) calculations found a weakly bound structure, (CH3)2SOH•, with a long S-O bond that was a local energy minimum. Single point calculations at the effective QCISD(T)/6-311++G(3df,2p) level of theory, denoted as G2++(MP2), found the (CH3)2S-OH• bonding energy to be 40 kJ mol-1 at 298 K. The standard heat of formation of (CH3)2SOH• was assessed from dissociation and isodesmic reactions as -45 ± 4 kJ mol-1. No other local minima corresponding to C2H7OS radicals were found at the present level of theory that could be derived from DMS or dimethyl sulfoxide (DMSO). A very weak complex, CH3S(H)-•OCH3, was found that was bound by mere 4 kJ mol-1 against dissociation to CH3SH and •OCH3. Vertical electron capture by (CH3)2SOH+ is predicted to form (CH3)2SOH• with a highly non-relaxed geometry corresponding to a vibrational excitation of 138 kJ mol-1 above the local minimum and 88 kJ mol-1 above the dissociation threshold to DMS and OH•. Unimolecular dissociation of (CH3)2SOH• to methanesulfenic acid (CH3SOH) and •OCH3 faces an energy barrier that diminishes at shorter S-O distances. The dipole-allowed electronic excitation in (CH3)2SOH• was calculated with CIS/6-311++G(2df,p) to have λmax = 248 nm in the gas phase. The resulting B state represents a charge-transfer complex of (CH3)2S+• and OH-. The present computational results allowed us to explain the existing controversy between the experimental results obtained by gas-phase flow kinetics, radiolysis in aqueous solution, and neutralization-reionization mass spectrometry.