Are the anions MeO(CO)n- (n = 1 and 2) methoxide anion donors in the gas phase? A theoretical investigation.

1. The anions CH(3)O-(-)CO and CH(3)OCO-(-)CO are both methoxide anion donors. The processes CH(3)O-(-)CO --> CH(3)O(-) + CO and CH(3)OCO-CO --> CH(3)O(-) + 2CO have DeltaG values of +8 and -68 kJ mol(-1), respectively, at the CCSD(T)/6-311++G(2d, 2p)//B3LYP/6-311++G(2d,2p) level of theory. 2. The reactions CH(3)OCOCO(2) (-) --> CH(3)OCO(2) (-) + CO (DeltaG = -22 kJ mol(-1)) and CH(3)COCH(O(-))CO(2)CH(3) --> CH(3)COCH(O(-))OCH(3) + CO (DeltaG = +19 kJ mol(-1)) proceed directly from the precursor anions via the transition states (CH(3)OCO...CO(2))(-) and (CH(3)COCHO...CH(3)OCO)(-), respectively. 3. Anion CH(3)COCH(O(-))CO(2)CH(3) undergoes methoxide anion transfer and loss of two molecules of CO in the reaction sequence CH(3)COCH(O(-))CO(2)CH(3) --> CH(3)CH(O(-))COCO(2)CH(3) --> [CH(3)CHO (CH(3)OCO-(-)CO)] --> CH(3)CH(O(-))OCH(3) + 2CO (DeltaG = +9 kJ mol(-1)). The hydride ion transfer in the first step is a key feature of the reaction sequence.

How does energized NCCCCCN lose carbon in the gas phase? A joint experimental and theoretical study.

Neutral NCCCCCN may be prepared in a collision cell of a VG ZAB 2HF mass spectrometer by charge stripping of (NCCCCCN)(*-), formed in the ion source by the process NCCCCH(OEt)(CN) + HO(-) --> H(2)O + NCCCC(-)(OEt)(CN) --> (NCCCCCN)(*-) + EtO(*). A comparison of the neutralization/reionization ((-)NR(+)) and charge reversal ((-)CR(+)) spectra of (NCCCCCN)(*-) indicate that some neutrals NCCCCCN are energized and rearrange to an isomer which decomposes by loss of carbon. An ab initio study at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G(3df) level of theory indicates that (i) triplet NCCCCCN is the ground state with a T/S energy gap of -14.9 kcal mol(-1); (ii) the structures of triplet and singlet NCCCCCN need to be described by molecular obital theory, and a simple valence bond approach cannot be used for this system; and (iii) there are several possible routes by which an energized neutral may lose carbon, but the major route involves the triplet nitrile to isonitrile rearrangement NCCCCCN --> CNCCCCN --> NCCCCN + C.

Anions [N(CH2)3]- and [ON(CH2)2]- are stable in the gas phase, but can they be charge stripped to form the radicals N(CH2)3 and ON(CH2)2? A joint experimental and theoretical study.

Collision-induced activation of deprotonated trimethylamine N-oxide yields the two anions [N(CH(2))(3)](-) and [ON(CH(2))(2)](-) following losses of H(2)O and CH(4), respectively. These two anions decompose by minor losses of H(*) and H(2) when collisionally activated: no other fragmentations are noted. Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory indicate that these trigonal anions are stable, and should not rearrange following collisional activation. Collisional-induced charge stripping of the anions [N(CH(2))(3)](-) and [ON(CH(2))(2)](-), respectively, form N(CH(2))(3) and ON(CH(2))(2). Some of these neutrals are energised and undergo rearrangement and dissociation. From a consideration of experiment and theory, it is proposed (i) that energised N(CH(2))(3) may cyclise to form the 1-aziridinylcarbinyl radical. This species may ring open to CH(2)=NCH(2)CH(2) which then decomposes to CH(2)N and C(2)H(4) and (ii) energised ON(CH(2))(2) may undergo OC cyclisation followed by ring opening to energised CH(2)=NCH(2)O which may fragment to yield CH(2)N and CH(2)O.

Is the hypothiocyanite anion (OSCN)- the major product in the peroxidase catalyzed oxidation of the thiocyanate anion (SCN)-? A joint experimental and theoretical study.

The hypothiocyanate anion (OSCN)(-) is reported to be a major product of the lactoperoxidase/H(2)O(2)/(SCN)(-) system, and this anion is proposed to have significant antimicrobial properties. The collision induced (CID) negative ion mass spectrum of "(OSCN)(-)" has been reported: there is a pronounced parent anion at m/z 74, together with fragment anions at m/z 58 (SCN)(-) and 26 (CN)(-). These fragment anions are consistent with structure (OSCN)(-). However there is also a lesser peak at m/z 42 (OCN(-) or CNO(-)) in this spectrum which is either formed by rearrangement of (OSCN)(-) or from an isomer of this anion. The current theoretical investigation of (OSCN)(-) and related isomers, together with the study of possible rearrangements of these anions, indicates that ground-state singlet (OSCN)(-) is a stable species and that isomerization is unlikely. The three anions (OSCN)(-), (SCNO)(-), and (SNCO)(-) have been synthesized (in the ion source of a mass spectrometer) by unequivocal routes, and their structures have been confirmed by a consideration of their collision induced (negative ion) and charge reversal (positive ion) mass spectra. The CID mass spectrum of (SCNO)(-) shows formation of m/z 42 (CNO(-)), but the corresponding spectra of (OSCN)(-) or (SNCO)(-) lack peaks at m/z 42. Combined theoretical and experimental data support earlier evidence that the hypothiocyanite anion is a major oxidation product of the H(2)O(2)/(SCN)(-) system. However, the formation of m/z 42 in the reported CID spectrum of "(OSCN)(-)" does not originate from (OSCN)(-) but from another isomer, possibly (SCNO)(-).

One-electron oxidation of [CCOCC]-* in the gas phase forms stable and decomposing forms of CCCCO.

The radical anion [CCOCC]-* may be made in the source of a VG ZAB 2HF mass spectrometer by the reaction between F-(from SF6) and (CH3)3SiC[triple bond]COC[triple bond]CSi(CH3)3. Vertical (Franck-Condon) one-electron oxidation of [CCOCC]-* in the first collision cell produces both singlet and triplet CCOCC. A combination of experiment and molecular modelling (at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* level of theory) gives data which are consistent with the CCOCC neutrals rearranging over small barriers to form singlet and triplet CCCCO in exothermic reactions. Both singlet and triplet CCCCO formed in this way have excess energy. Singlet CCCCO has sufficient excess energy to effect decomposition exclusively to CCC and CO. In contrast, some of the triplet CCCCO neutrals are stable, while others decompose to CCC and CO.

Generation of transient neutrals in the gas phase from anionic precursors. Does energised CNCCO rearrange to NCCCO?

The stability and reactivity of the neutral species CNCCO generated by one electron oxidation of the anion [CNCCO](-) have been investigated by a combination of theoretical calculations (carried out at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory) and tandem mass spectrometric experiments. Some of the neutrals formed in this way are stable for the microsecond duration of the experiment, but others are energised. The neutrals which are energised may either (i) dissociate [CNCCO --> CNC + CO (+92 kJ mol(-1))], and/or (ii), undergo the isonitrile to nitrile rearrangement to yield NCCCO energised neutrals (barrier 133 kJ mol(-1), reaction exothermic by 105 kJ mol(-1)). Some of these rearranged neutrals NCCCO have excess energies as high as 238 kJ mol(-1) and will dissociate [NCCCO --> NCC + CO (+203 kJ mol(-1))].

The formation of RCCCO and CCC(O)R (R = Me, Ph) neutral radicals from ionic precursors in the gas phase: the rearrangement of CCC(O)Ph.

Neutrals MeCCCO, CCC(O)Me, PhCCCO and CCC(O)Ph have been made by neutralisation of [MeCCCO](+), [CCC(O)Me](-), [PhCCCO](+) and [CC(CO)Ph](-). Neutrals MeCCCO, CCC(O)Me and PhCCCO are stable for the microsecond duration of the neutralisation experiment. A joint experimental and theoretical study (energies calculated at the B3LYP/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory) suggests that the neutral radical CCC(O)Ph rearranges via a four-centred ipso radical cyclisation/ring opening to form the isomer PhCCCO in an exothermic reaction. (13)C labelling confirms that the rearrangement does not involve O migration. Some of the PhCCCO radicals formed in this reaction are sufficiently energised to effect decomposition to give PhCC and CO.

Gas phase generation of the neutrals H2CCCCO, HCCCCDO and CCCHCHO from anionic precursors. Rearrangements of HCCCCDO and CCCHCHO. A joint experimental and theoretical study.

The reaction between O-. and MeO-CH2-C identical to C-CDO in the ion source of a VG ZAB 2HF mass spectrometer gives a number of product anions including [H2CCCCO]-. and [HCCCCDO]-. (in the ratio 1:5). Neutralisation-reionisation (NR+) of [H2CCCCO]-. results in the sequential two-electron vertical oxidation [H2CCCCO]-.-->H2CCCCO-->[H2CCCCO](+.). Singlet H2CCCCO lies 158 kJ mol-1 below the triplet [at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory]. The majority of neutrals H2CCCCO are stable for the microsecond duration of the NR experiment, but some are energized and decompose to give H2CCC and CO. A similar NR+ experiment with [HCCCCDO]-. yields neutrals HCCCCDO, some of which are excited and rearrange. Calculations show that it is the singlet form of HCCCCHO which rearranges (the singlet lies 36 kJ mol-1 above the ground state triplet): the rearrangement occurs by the sequential H transfer process, HCCCCHO-->HCC(CH)CO<--H2CCCCO. Neutral HCCCCHO needs an excess energy of only 43 kJ mol-1 to effect this reaction, which is exothermic by 230 kJ mol-1. Both HCC(CH)CO and H2CCCCO formed in this way should have sufficient excess energy to cause some loss of CO. The anions [CC(CH)CHO]-. and [CC(CD)CHO]-. are formed in the ion source of the mass spectrometer by the reactions of HO- with Me3SiC identical to C-CH = CHOMe and Me3SiC identical to C-CD = CHOMe respectively. NR+ of these anions indicate that energized forms of CC(CH)CHO and CC(CD)CHO may rearrange to isomer(s) which decompose by loss of CO. Singlet CC(CH)CHO rearranges to HCC(CH)CO and H2CCCCO, both of which are energized and fragment by loss of CO.

Generation of neutrals from ionic precursors in the gas phase. The rearrangement of CCCCCHO to HCCCCCO.

The neutrals HCCCCCO and CCCCCHO have been studied by experiment and by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. Neutral HCCCCCO has been made by one-electron reduction of [HCCCCCO]+ in the dual collision cell of a VG ZAB 2HF mass spectrometer. The isomer CCCCCHO is also formed in the dual collision cell, but this time by one-electron oxidation of the anion [CCCCCHO]-. Comparison of the CID and +NR+ mass spectra of [HCCCCCO]+ indicates that neutral HCCCCCO, when energised, retains its structural integrity. If the excess energy of HCCCCCO is > or = 170 kJ mol-1, decomposition can occur to give HCCCC and CO (calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory). The situation with the isomer CCCCCHO is different. Comparison of the -CR+ and -NR+ spectra of [CCCCCHO]- shows that both neutral and cationic forms of CCCCCHO partially rearrange to a species which decomposes by loss of CO. The peak corresponding to loss of CO is more pronounced in the -NR+ spectrum, indicating that the rearrangement is more prevalent for the neutral than the cation. Theoretical calculations suggest that the species losing CO could be CCCCHCO or HCCCCCO, but that HCCCCCO is the more likely. The lowest-energy rearrangement pathway occurs by successive H transfers, namely CCCCCHO-->CCCCHCO-->CCCHCCO-->HCCCCCO. The rearrangement of CCCCCHO to HCCCCCO requires CCCCCHO to have an excess energy of > or = 94 kJ mol-1. The species HCCCCCO formed by this exothermic sequence (214 kJ mol-1) has a maximum excess energy of 308 kJ mol-1: this is sufficient to effect decomposition to HCCCC and CO.

Formation of neutral molecules of potential stellar interest by neutralisation of negative ions in a mass spectrometer. The application of experiment and molecular modelling in concert.

This paper is a modified version of a lecture which describes the synthesis, structure and reactivity of some neutral molecules of stellar significance. The neutrals are formed in the collision cell of a mass spectrometer following vertical Franck-Condon one electron oxidation of anions of known bond connectivity. Neutrals are characterised by conversion to positive ions and by extensive theoretical studies at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. Four systems are considered in detail, viz (i) the formation of linear C(4) and its conversion to the rhombus C(4), (ii) linear C(5) and the atom scrambling of this system when energised, (iii) the stable cumulene oxide CCCCCO, and (iv) the elusive species O(2)C-CO. This paper is not intended to be a review of interstellar chemistry: examples are selected from our own work in this area.

Collision-induced fragmentations of the (M-H)- parent anions of underivatized peptides: an aid to structure determination and some unusual negative ion cleavages.

This article describes the fundamental cleavage reactions of (M-H)(-) anions of underivatized peptides that contain up to 25 amino acid residues. The experimental observations of these cleavages have been backed up by molecular modeling, generally at the AM1 level of theory. The basic cleavages are the ubiquitous alpha- and beta-backbone cleavage reactions, which provide information similar to that of the B and Y + 2 cleavages of MH(+) ions of peptides. The residues Asp and Asn also effect cleavages of the backbone (called delta- and gamma-cleavages), by reactions initiated from side chain enolate anions, causing elimination reactions that cleave the backbone between the Asp (Asn) N bond;C backbone bond. Glu and Gln also direct analogous delta- and gamma-cleavages of the backbone, but in this case the processes are initiated by attack of the side chain CO(2) (-) (CONH(-)) to form a lactone (lactam). Ser and Thr residues undergo characteristic fragmentations of the side chain. These processes, losses of CH(2)O (Ser) and MeCHO (Thr), convert these residues into Gly. In larger peptides, Ser and Thr can effect two backbone cleavage reactions, called gamma- and epsilon -processes. The C-terminal CO(2) (-) (or CONH(-)) forms a hydrogen bond with the side chain OH (of Ser or Thr), placing the C-terminal residue in a position where it may affect S(N) (2) attack at the electrophilic backbone CH of Ser, with concomitant cleavage of the backbone. All of the above negative ion cleavages require the peptide backbone to be conformationally flexible. However, there is a backbone cleavage that requires the peptide to have an alpha-helical conformation in order for the two reacting centers to approach. This cleavage is illustrated for the Glu 23-initiated backbone cleavage at Ile 21 for the (M-H)(-) anion of the antimicrobial peptide caerin 1.1.

Backbone cleavages of [M - H](-) anions of peptides. Cyclisation of citropin 1 peptides involving reactions between the C-terminal [CONH](-) residue and backbone amide carbonyl groups. A new type of beta cleavage: a joint experimental and theoretical study.

This paper reports the study of backbone cleavages in the collision-induced negative-ion mass spectra of the [M - H](-) anions of some synthetic modifications of the bioactive amphibian peptide citropin 1 (GLFDVIKKVASVIGGL-NH(2)). The peptides chosen for study contain no amino acid residues which could effect facile side-chain cleavage, i.e. Ser (-CH(2)O, side-chain cleavage) and Asp (-H(2)O) are replaced by Ala or Lys. We expected that such peptides should exhibit standard and pronounced peaks due to alpha cleavage ions (and to a lesser extent beta cleavage ions) in their collision-induced negative-ion spectra. This expectation was realised, but the spectra also contained peaks formed by a new series of cleavage anions. These are produced following cyclisation of the C-terminal CONH(-) moiety at carbonyl functions of amide groups along the peptide backbone; effectively transferring the NH of the C-terminal CONH(-) group to other amino acid residues. We have called the product anions of these processes beta' ions, in order to distinguish them from standard beta ions. Some beta' ions also fragment directly to some other beta' ions of smaller mass. The reaction coordinates of alpha,beta and beta' backbone processes have been calculated at the HF/6-31G*//AM1 level theory for simple model systems. The initial cyclisation step of the beta' sequence is barrierless and exothermic. Subsequent steps have a maximum barrier of +40 kcal mol(-1), with the overall reaction being endothermic by some 30 kcal mol(-1) at the level of theory used. These calculations take no account of the complexity of the conformationally flexible peptide system, and it is surprising that each of the two reacting centres can 'find' each other in such a large system.