Computational chemistry: A useful (sometimes mandatory) tool in mass spectrometry studies

Alcamı́, Manuel; Mó, Otília; Yáñez, Manuel

doi:10.1002/mas.10005

Cited by 170 publications

(157 citation statements)

References 287 publications

(318 reference statements)

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“…We expect that the present paper will stimulate experimental work in this field of ultrastrong neutral Brønsted superacids, in particular that it will promote syntheses of suitably substituted cyclopentadienes. We hope that theoretical methods will continue to provide help in this respect since computational chemistry is almost a mandatory tool in contemporary acid/base chemistry, at least in the gas phase, as pointed out by YµÇez et al [19] To summarize, the most acidic compounds studied here are 2(1), 4, 2(2), and 5. The last two species are more of theoretical than practical interest in view of their very high energy density and concomitant hazardous behavior, which probably preclude laboratory applications.…”

mentioning

confidence: 78%

In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

Vianello

Liebman

Maksić

2004

Chemistry A European J

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An efficient but reasonably accurate B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) computational procedure showed that pentasubstituted cyclopentadienes such as (CN)5C5H, (NO2)5C5H, and (NC)5C5H containing strongly electron-withdrawing groups are neutral organic superacids of unprecedented strength. The boldface denotes the atom attached to the cyclopentadiene framework. All of them exhibit prototropic tautomerism by forming somewhat more stable structures with C=NH, NO2H, and N=CH exocyclic fragments, respectively. The acidity (DeltaH(acid)) of these is lower, but only to a rather small extent. The DeltaH(acid) enthalpies of these last three tautomers are estimated to be 271, 276, and 282 kcal mol(-1), respectively. Hence, the most stable tautomers of (CN)5C5H and (NC)5C5H represent a legitimate target for synthetic chemists. On the other hand, (NO2)5C5H is less suitable for practical applications, because of its high energy density. The origin of the highly pronounced acidity of these compounds was analyzed by using the recently developed triadic formula. It is found that very high Koopmans' ionization energy (IE)n(Koop) of conjugate bases exerts a decisive influence on acidity. It follows as a corollary that the overwhelming effect leading to very high acidity is due to the properties of the final state. An alternative picture is offered by homodesmotic reactions, wherein the cyclic systems are compared with their linear counterparts. It is found that the acidity of cyclopentadiene (CP) is a consequence of aromatic stabilization in the CP- anion. The extreme acidity of pentacyanocyclopentadiene (CN)5C5H is due to aromatization of the five-membered ring and a strong anionic resonance effect in the resultant conjugate base. The neutral organic superacids predicted by the present calculations may help to bridge the gap between existing very strong acids and bases.

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confidence: 78%

In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

Vianello

Liebman

Maksić

2004

Chemistry A European J

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“…Posteriormente, determina-se a estrutura eletrônica e as grandezas termoquímicas do íon formado (ou de um íon precursor de interesse), traçando-se 40,41 A popularização dos modelos químico-quânticos devido ao desenvolvimento de novos métodos computacionais, além do incremento do poder computacional e do barateamento dos computadores são alguns dos fatores responsáveis por esse progresso. 4 De fato, muitas idéias da química moderna resultaram da combinação entre técnicas experimentais e cálculos teóricos. Nos últi-mos anos, a química quântica tem se tornado uma área de interesse multidisciplinar, tendo encontrado aplicação em diferentes áreas da ciência, incluindo a espectrometria de massas.…”

Section: Aspectos Mecanísticos Das Reações De Fragmentaçãounclassified

Section: Introductionunclassified

“…4 No caso de substâncias que apresentam vários grupos funcionais ácidos e/ou básicos em suas estruturas, muitas vezes é difícil se identificar qual é o sítio mais suscetível à protonação ou à desprotonação, 5 mesmo existindo na literatura extensas compilações sobre acidez e basicidade em fase gasosa. 6 Em princípio, a suscetibilidade à protonação apresentada por dois grupos funcionais diferentes em uma mesma estrutura quími-ca pode ser estimada a partir da diferença entre suas basicidades em fase gasosa.…”

Section: Introductionunclassified

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Aplicação da química quântica computacional no estudo de processos químicos envolvidos em espectrometria de massas

Vessecchi¹,

Galembeck²,

Lopes³

et al. 2008

Quím. Nova

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Recebido em 4/7/07; aceito em 14/11/07; publicado na web em 19/3/08 APPLICATION OF COMPUTATIONAL QUANTUM CHEMISTRY TO CHEMICAL PROCESSES INVOLVED IN MASS SPECTROMETRY. The field of application of mass spectrometry (MS) has increased considerably due to the development of ionization techniques. Other factors that have stimulated the use of MS are the tandem mass spectrometry (MS/MS) and sequential mass spectrometry (MS n ) techniques. However, the interpretation of the MS/MS and MS n data may lead to speculative conclusions. Thus, various quantum chemical methods have been applied for obtaining high quality thermochemical data in gas phase. In this review, we show some applications of computational quantum chemistry to understand the formation and fragmentation of gaseous ions of organic compounds in a MS analysis.Keywords: theoretical calculations; fragmentation studies; protonation site. INTRODUÇÃONo século passado, vários métodos e técnicas para obtenção de parâmetros moleculares aplicando a química quântica a sistemas multi-eletrônicos foram desenvolvidos. Este desenvolvimento foi fortemente impulsionado pelo aumento do poder computacional, conforme previsto pela Lei de Moore, 1 bem como pelo desenvolvimento de novos métodos computacionais e aperfeiçoamento daqueles já existentes. Atualmente, parâmetros como a energia de um sistema em uma dada geometria e freqüências vibracionais podem auxiliar na determinação da estrutura e das propriedades químicas de intermediários e de possíveis estados de transição para um dado caminho de reação, além de assistirem na obtenção de correlações entre as entalpias de reação e a temperatura, efeitos isotópicos e energias de dissociação. Comumente, nos estudos químico-quântico computacionais utiliza-se como modelo uma molécula isolada no vácuo. Há, portanto, certas similaridades entre estas condições e as condições de baixas pressões no interior de um espectrômetro de massas, onde ocorrem vários fenômenos em fase gasosa.3 Neste contexto, é possível utilizar estes métodos para se estimar uma série de parâmetros de interesse de um espectroscopista de massas, auxiliando na compreensão tanto dos fenômenos de geração de íons como nos processos de fragmentação dos mesmos.Em processos de ionização, por exemplo, é possível obter parâmetros como a energia de ionização adiabática, basicidade e afinidade protônica, entre outros. 4 No caso de substâncias que apresentam vários grupos funcionais ácidos e/ou básicos em suas estruturas, muitas vezes é difícil se identificar qual é o sítio mais suscetível à protonação ou à desprotonação, 5 mesmo existindo na literatura extensas compilações sobre acidez e basicidade em fase gasosa.6 Em princípio, a suscetibilidade à protonação apresentada por dois grupos funcionais diferentes em uma mesma estrutura quími-ca pode ser estimada a partir da diferença entre suas basicidades em fase gasosa. Entretanto, quando os grupos funcionais são parecidos, tal como os nitrogênios de cadeia lateral e de anel de histamina ou pirimidinas, a identificação do sítio m...

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“…Methods for determining absolute values of thermochemical quantities are rarely applicable, so relative values are typically measured by kinetic and equilibrium measurements on gas-phase ions. Theoretical calculations serve a special reference function [4]. Under appropriate conditions, the kinetics of the dissociation of molecular cluster ions can yield relative, but quantitative, thermochemical information on the constituent species [5].…”

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confidence: 99%

Alkali chloride cluster ion dissociation examined by the kinetic method: Heterolytic bond dissociation energies, effective temperatures, and entropic effects

Denault

Cooks

et al. 2002

J. Am. Soc. Mass Spectrom.

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Branching ratios have been measured as a function of collision energy for the dissociation of mass-selected chloride-bound salt cluster ions, Cl-M i ] ϩ , where M i ϭ Na, K, Cs. The extended version of the kinetic method was used to determine the heterolytic bond dissociation energy (HBDE) of Rb-Cl. The measured value of 480.8 Ϯ 8.5 kJ/mol, obtained under single collision conditions, agrees with the HBDE value (482.0 Ϯ 8.0 kJ/mol), calculated from a thermochemical cycle. The observed effective temperature of the collisionally activated salt clusters increases with laboratory-frame collision energy under both single-and multiplecollision conditions. Remarkably, the effective temperatures under multiple collision conditions are lower than those recorded under single-collision conditions at the same collision energy, a consequence of the inability of the triatomic ions to store significant amounts of internal energy. Laboratory-frame kinetic energy to internal energy transfer (T3 V) efficiencies range from 3.8 to 13.5%. For a given cluster ion, the T3 V efficiency decreases with increasing collision energy. Many features of the experimental results are accounted for using . Not only is this information useful in the interpretation and prediction of ionic reactivities and ion/molecule mechanisms, but the paucity of alternative sources of this information has made its acquisition an important application of ion/molecule reactions [2,3]. Methods for determining absolute values of thermochemical quantities are rarely applicable, so relative values are typically measured by kinetic and equilibrium measurements on gas-phase ions. Theoretical calculations serve a special reference function [4]. Under appropriate conditions, the kinetics of the dissociation of molecular cluster ions can yield relative, but quantitative, thermochemical information on the constituent species [5]. The kinetic method, based on this concept, has been widely used for estimating thermochemical properties [5,6]. Cluster ions bound via protons (eq 1), as well as other atomic or polyatomic anions or cations can be isolated and their dissociations followed in a tandem mass spectrometry experiment [7]. In eq 1, k 1 and k 2 represent the rate constants for two competitive dissociation channels of the proton-bound dimer to yield B 1 H ϩ and B 2 H ϩ , respectively. The ratio of rate constants, viz. the branching ratio of two fragment ion abundances, is related logarithmically to the gas-phase basicities of B 1 and B 2 , in the standard form of the kinetic method:In eq 2, ⌬(⌬G) is the difference in gas-phase basicities of B 1 and B 2 , R is the gas constant, and T eff is the effective temperature (the parameter in the kinetic method which connects the degree of fragmentation of

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Computational chemistry: A useful (sometimes mandatory) tool in mass spectrometry studies

Cited by 170 publications

References 287 publications

In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

In Search of Ultrastrong Brønsted Neutral Organic Superacids: A DFT Study on Some Cyclopentadiene Derivatives

Aplicação da química quântica computacional no estudo de processos químicos envolvidos em espectrometria de massas

Alkali chloride cluster ion dissociation examined by the kinetic method: Heterolytic bond dissociation energies, effective temperatures, and entropic effects

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