Journal of Molecular Structure 981 (2010) 153–158

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Journal of Molecular Structure

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Dissociation of the NAO bond vs. NAC bond in Me3NOX+ (X = H, Li, Na and K):An experimental and theoretical study

Xiang ZhangDepartment of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 June 2010Received in revised form 28 July 2010Accepted 28 July 2010Available online 3 August 2010

Keywords:Trimethylamine oxideNAO bondNAC bondDFTMS/MS

0022-2860/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.molstruc.2010.07.047

E-mail address: zhangxiang@mail.zjgsu.edu.cn

The dissociation of the NAO bond and NAC bond in the protonated trimethylamine oxide (Me3NOH+) andin adducts of alkali metal cations (Li+, Na+ and K+) and the trimethylamine oxide is investigated throughthe density functional theory (DFT) and tandem mass spectrometry. DFT calculations show that due tothe introduction of the cations (H+, Li+, Na+, K+), the NAC bond is strengthened and the correspondingbond dissociation energy follows the order of Me3NO < Me3NOK+ < Me3NONa+ < Me3NOLi+ < Me3NOH+,while the NAO bond is weakened and the corresponding bond dissociation energy follows the order ofMe3NO > Me3NOK+ > Me3NONa+ > Me3NOLi+ > Me3NOH+. In addition, in Me3NO, Me3NOK+, Me3NONa+

and Me3NOLi+, the dissociation of the NAC bond has precedence over that of the NAO bond, but inMe3NOH+, the dissociation of the NAO bond is easier to take place. The MS/MS experiments are consis-tent with the DFT calculations. On the one hand, the fragment ion at m/z 59 in the CID spectrum ofMe3NOH+ indicates the dissociation of the NAO bond. On the other hand, the fragment ion at m/z 83in that of Me3NONa+ manifests the dissociation of NAC bond, while in the CID spectra of the othertwo adducts (Me3NOK+ and Me3NOLi+), there exists no corresponding fragment ion to indicate the disso-ciation of NAC bond. The reason might be as follows: according to the calculations, in Me3NOK+ the OAKbond is weaker than the NAC bond and thus it’s easier to break, and in Me3NOLi+, as all chemical bondsare very strong, its dissociation might be difficult to take place in MS/MS experiments.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

It is well known that the N-oxides of aliphatic amines occurextensively in biological systems. It is suggested that in plantsamine oxides are the intermediates in the synthesis of alkaloids[1]. Tracer studies have proved that ‘‘Berberine Bridge” comes fromthe tertiary amine via N-oxide [2,3]. In Horning’s group, it has beenreported that liver homogenates can oxidize tertiary amines toamine oxides and cleave N-oxides to secondary amines and alde-hydes. At the same time, Ferris et al. pointed out that in the cleav-age of trimethylamine oxide to the formaldehyde, trimethylamineradical cation was the key intermediate [4–8].

Recently the formaldehyde pollution of sea food has arousedgreat attention and interest. Most literature ascribes the formationof formaldehyde to the catalysis of endogenous TMAOase enzymeson trimethylamine oxide. Presumably these enzymes directly uti-lize the trimethylamine oxide as a substrate to form formaldehyde[9–13]. On the other hand, the pyrolysis of trimethylamine oxidecan also lead to the formation of formaldehyde with the aid ofreductive substance [7]. As for the latter formation of formalde-hyde, trimethylamine radical cation has been proposed to be the

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key intermediate. It’s obvious that the dissociation of NAO bondin trimethylamine oxide is ineluctable in either formation offormaldehyde.

A lot of facts have proved that the tandem mass spectrometrytechnique is a perfect tool to study the cleavage of the chemicalbond in organic molecules. MS/MS not only provides extensivestructural information about organic molecules, but also can actas an important tool for mechanistic studies in gas-phase organicchemistry [14–23]. In this paper, the trimethylamine oxide is se-lected as the target compound to study the dissociation of pivotalbonds. Through the tandem mass spectrometry, the investigationhas been conducted into the dissociation of chemical bonds inthe protonated trimethylamine oxide and in adducts of alkali metalcations (Li+, Na+ and K+) and the trimethylamine oxide. The proba-ble dissociation mechanism is depicted in Scheme 1. Two cleavagepathways are included. One is the dissociation of the NAO bondwhich gives rise to the formation of trimethylamine radical cationand �OX (X = H, Li, Na and K); the other is the dissociation of theNAC bond that leads to the formation of methyl radical and(CH3)2NOX+� (X = H, Li, Na and K). Since these two kinds of dissoci-ation occur in the transformation of even electron compounds toodd electron compounds, the MS/MS experiments would be a littledifficult to carry out.

N+ OX

N+ + OX

N+OX

+ CH3

X = H, Li, Na, K

Scheme 1. Probable dissociation pathway of (CH3)3NOX+ (X = H, Li, Na and K).

154 X. Zhang / Journal of Molecular Structure 981 (2010) 153–158

Besides, Density Functional Theory (DFT) [24,25] has been ap-plied to the investigation into the interactions between alkali me-tal cations (Li+, Na+, K+) and trimethylamine oxide (TMAO). Asreported in the literature, DFT has been widely used to study theinteractions of alkali metal cations with many compounds, suchas hydroxyl side-chain amino acids, N-methylaniline and triethylphosphate, and the theoretical calculations of the correspondingbond energy are in good agreement with the experimental values[26,27]. At the same time, the influence of the proton and alkalimetal cations upon the NAO bond and NAC bond will be given athorough study on the basis of DFT.

2. Experimental

2.1. Material

Trimethylamine oxide (TMAO) was obtained from the Sigma–Aldrich Company. Analytic grade lithium chloride, sodium chlo-ride, and potassium chloride were purchased from Shanghai Exper-iment Reagent Co., Ltd.

2.2. Mass spectrometry

The mass spectral data were acquired on a LCQ ion trap massspectrometer from ThermoFinnigan (San Jose, CA, USA) equippedwith an electrospray ionization (ESI) interface operated in the po-sitive ion mode. The operation parameters are listed as follows:spray tip potential 4830 V; capillary voltage: 20 V; capillary tem-perature: 240 �C; Sheath gas flow rate: 9.28 arbitrary units; Tubelens: 55 V. In MS/MS experiments, the protonated trimethylamineoxide and adducts of trimethylamine oxide and a series of alkalimetals have been isolated monoisotopically in the ion trap and col-lisionally activated by different collision energies so as to find theoptimal collision energies for a distinct fragmentation. Helium(99.99%) was used as the trapping and collision gas. The collisionenergy ranged from 10 to 40 eV (%), depending on the dissociationlability of the precursor ion.

2.3. Computational methods

All structures were computed on the basis of the hybrid densityfunctional theory ((U)B3LYP) [28] and the 6-31 + G(d, p) basis setwhich were implemented in Gamess 09 [29,30] program package.All the gas phase minima were characterized by frequency analy-sis. Frequency calculations identify minimum structures with allreal frequencies. The zero-point energy (ZPE) corrections were ob-tained at the same level of theory. Spin contamination for all of thestationary points was minimal (i.e., 0.75 < S2 < 0.77). The computa-tional structures were displayed with MacMolPlt v7.4 [31].

3. Results and discussion

Our work started from the MS/MS experiment of the protonatedtrimethylamine oxide, and the CID spectrum is depicted in Fig. 1.Two fragment ions were observed: one at m/z 59 and the otherat m/z 58. They come from the loss of �OH and H2O respectivelyfrom the protonated trimethylamine oxide. However, the fragmention at m/z 61 from the loss of �CH3 has not been found. In otherwords, in the protonated trimethylamine oxide, the dissociationof the NAO bond has absolute precedence over that of the NACbond. Next, MS/MS experiments of adducts of alkali metal cations(Li+, Na+, K+) and trimethylamine oxide (TMAO) have been carriedout. The results show that only adduct (CH3)3NONa+ could lose�CH3 to produce the radical cation (CH3)2NONa+�, and the corre-sponding CID spectrum is depicted in Fig. 2. In this figure, the frag-ment ion at m/z 83 resulting from the dissociation of NAC bond isshown; however, the fragment ion at m/z 81 from the dissociationof NAO bond does not occur. As for adducts (CH3)3NOLi+ and(CH3)3NOK+, neither the dissociation of the NAO bond nor that ofthe NAC bond has been found. According to the results of theMS/MS experiments, the dissociation of the NAO bond and theNAC bond in cations (CH3)3NOX+ (X = H, Li, Na and K) are summa-rized in Scheme 2.

It is obvious that in the protonated trimethylamine oxide andadducts of alkali metal cations (Li+, Na+ and K+) and trimethyla-mine oxide, there exists great difference in the dissociation of theNAO bond and that of the NAC bond. An attempt will be madeto account for this phenomenon on the basis of the density func-tional theory (DFT). Fig. 3 shows the structures of the protonatedtrimethylamine oxide and adducts of alkali metal cations and tri-methylamine oxide which were optimized at B3lyp/6-31 + g(d, p)level. First of all, the interaction between the alkali metal cationsand oxygen atom in trimethylamine oxide has been investigated.Generally, there are two kinds of interactions: one is the electro-static interaction; the other is the formation of the direct metal–oxygen chemical bond. Lots of literature has reported the electro-static interaction between the alkali metal cations and the oxygenatom, which was investigated either theoretically or experimen-tally. For example, in the complex of the alkali metal cations and1,2-dimethoxyethane [32], the metal–oxygen distances optimizedat MP2/6-31 + g(d) level are: 1.889 (Li), 2.232 (Na), 2.643 (K), whilethose optimized at MP2/aug-cc-pVDZ level are: 1.901 (Li), 2.251(Na), 2.653 (K) respectively.

In the systems of this paper, the metal–oxygen distances opti-mized at B3lyp/6-31 + g(d, p) level are: 1.372 (Li), 2.052 (Na),2.329 (K), much shorter than the corresponding ones in the com-plexes of alkali metal cations and 1,2-dimethoxyethane. At thesame time, the calculated and experimental values of the XAObond lengths in XOH, X2O and XO2 (X = Li, Na and K) are listed inTable 1. It has been found that in present system, the XAO bonddistances between the alkali metal cations and the oxygen atomin trimethylamine oxide are very close to those in XOH, X2O andXO2 systems, which indicates that a direct XAO (X = Li, Na and K)chemical bond has formed between the alkali metal cations andthe oxygen atom in trimethylamine oxide. In addition, accordingto DFT calculations, the OAH distance in the protonated trimethyl-amine oxide is 0.976 Å, close to that in H2O. Therefore, in the pro-tonated trimethylamine oxide and the adducts of alkali metalcations (Li+, Na+, K+) and trimethylamine oxide, chemical bondshave formed between cations (H+, Li+, Na+, K+) and oxygen atom,not just the electrostatic interaction.

Next, the focus will be on the different influences of these cat-ions (H+, Li+, Na+, K+) upon the NAO bond and the NAC bond. As de-picted above, in the protonated trimethylamine oxide, thedissociation of the NAO bond has great precedence over that of

Fig. 2. ESI-CID MS/MS spectrum of adduct (CH3)3NONa+ at m/z 98.

Fig. 1. ESI-CID MS/MS spectrum of protonated trimethylamine oxide at m/z 76.

X. Zhang / Journal of Molecular Structure 981 (2010) 153–158 155

the NAC bond; however, in adduct (CH3)3NONa+, there appears thesituation to the contrary. As for adducts (CH3)3NOLi+ and(CH3)3NOK+, the dissociation of the NAO bond and the NAC bondseems very difficult. As for this interesting phenomenon, DFT cal-culations of the bond dissociation energy (BDE) of the NAO bondand NAC bond could provide reasonable explanation.

Table 2 shows corresponding BDEs of the NAO bond and NACbond in complexes (CH3)3NOX+ (X = H, Li, Na and K) computed at(U)B3lyp/6-31 + g(d, p) level, together with the zero-point energy(ZPE) corrections at the same level. At the same time, dissociationenergies of the OAX (X = H, Li, Na and K) bond are given withoutBSSE corrections. Firstly, for the protonated trimethylamine oxide,

N+ O

H

N+ + OH

N+OH

+ CH3m/z 76

m/z 59 N+ O

Na

N+ + ONa

N+ONa

+ CH3m/z 98

m/z 83

N+ O

Li

N+ + OLi

N+OLi

+ CH3m/z 78

N+ O

K

N+ + OK

N+OK

+ CH3m/z 114

Scheme 2. Dissociation of the NAO bond and the NAC bond in cations (CH3)3NOX+ (X = H, Li, Na and K).

Fig. 3. Structures of the protonated trimethylamine oxide and adducts of alkalimetal cations (Li+, Na+, K+) and trimethylamine oxide optimized at B3lyp/6-31 + g(d,p) level. (Å).

156 X. Zhang / Journal of Molecular Structure 981 (2010) 153–158

the bond dissociation energy of NAO bond is 47.6 kcal/mol, lessthan that of the NAC bond by 19.1 kcal/mol. Obviously, in the pro-tonated trimethylamine oxide, the dissociation of the NAO bondoccupies the absolute leading position. In adducts of alkali metalcations and trimethylamine oxide, it has been found that BDEs ofthe NAO bond are much bigger than those of the NAC bond, whichindicates that the dissociation of the NAC bond should be the maincleavage reaction. But, the dissociation of the NAC bond could onlybe observed in adduct (CH3)3NONa+. MS/MS spectra of adducts(CH3)3NOLi+ and (CH3)3NOK+ do not show any information aboutthe dissociation of the NAO bond or NAC bond. This prompts usto explore other possible reasons. Thus, the bond dissociation ener-gies of the OAX (X = Li, Na and K) bond are examined. In adduct

(CH3)3NOK+, BDE of the OAK bond is only 43.1 kcal/mol, less thanthat of the NAC bond by 1.4 kcal/mol. That is to say, compared tothe NAC bond, the dissociation of the OAK bond happens moreeasily during the CID experiment of adduct (CH3)3NOK+, which ex-plains the reason why neither the dissociation of the NAC bond northat of the NAO bond occurs. In adduct (CH3)3NONa+, among theBDEs of the NAC bond, NAO bond and OANa bond, BDE of theNAC bond is still the least, namely the dissociation of the NACbond is most favorable, which is consistent with the descriptionabove.

As for adduct (CH3)3NOLi+, the situation is special. Consideringthe order of the bond dissociation energy: BDENAO > BDEOALi >BDENAC, it could be deduced that the dissociation of the NAC bondis most probable. However, BDE of the NAC bond is as high as49.9 kcal/mol, more than that of the NAO bond in (CH3)3NOH+

and the NAC bond in adduct (CH3)3NONa+ by 2.3 and 4.1 kcal/mol, respectively. BDEs of the chemical bonds in adduct (CH3)3NO-Li+ are so large that the dissociation of chemical bonds could not becarried out in the MS/MS experiments.

Besides, BDEs of the NAO bond and NAC bond in the neutral tri-methylamine oxide are also worth discussing. DFT calculationsshow that BDE of the NAO bond is as high as 228.3 kcal/mol andBDE of the NAC bond is only 30.5 kcal/mol, as is shown on the leftside of Fig. 4. In addition, the trimethylamine oxide radical cationhas also been taken into consideration, which is shown on the rightside of Fig. 4. Calculations show that BDE of the NAC bond in tri-methylamine oxide radical cation is just 25.5 kcal/mol. As for theNAO bond, two situations should be taken into account. One is thatthe dissociation of the NAO bond leads to the formation of oxygenin the singlet state, whose process needs the energy of 107.2 kcal/mol. The other is that the dissociation of the NAO bond gives riseto the formation of oxygen in the triplet state, and the processneeds the energy of 42.0 kcal/mol. No doubt that the formationof oxygen in the triplet state is more favorable.

According to the DFT calculations, it is easy to deduce that in theneutral trimethylamine oxide, the dissociation of the NAC bondhas more advantage over that of the NAO bond and the dissocia-tion of the NAO bond seems very difficult. Fig. 5 is the comparison

Table 1Calculated and experimental data about the XAO bond lengths in XOH, X2O and XO2 systems in the literature (X = Li, Na and K) (Å).

OAX bond distance OALi OANa OAK

Calculated bond lengths in this work 1.372 2.052 2.329Calculated XAO bond lengths in XOH, X2O and XO2 systems

at CCSD(T)/awCVQZ level [33]1.580(LiOH) 1.939(NaOH) 2.203(KOH)

1.615(Li2O) 1.983(Na2O) 2.228(K2O)1.748(LiO2) 2.124(NaO2) 2.402(KO2)

Experimental data about the XAO bond lengths in XOH, X2Oand XO2 systems [33]

1.580(LiOH) 1.950(NaOH) 2.212(KOH)

1.606(Li2O) 2.000(Na2O) 2.120(K2O)1.770(LiO2) 2.070(NaO2) 2.280(KO2)

Calculated metal–oxygen distances in the complexes of alkalimetal cations with 1,2-dimethoxyethane at different levels [32]

1.889(MP2/ 2.232(MP2/ 2.643(MP2/

6-31 + G(d)) 6-31 + G(d)) 6-31 + G(d))1.901(MP2/aug-cc-pVDZ) 2.251(MP2/aug-cc-pVDZ) 2.653(MP2/aug-cc-pVDZ)

Table 2Bond dissociation energies of the NAO bond, NAC bond and OAX bond in complexes(CH3)3NOX+ (X = H, Li, Na and K) (kcal/mol).

BDENAO BDENAC BDEXAO

(CH3)3NOH+ 47.6 66.7 74.3(CH3)3NOLi+ 83.6 49.9 70.1(CH3)3NONa+ 92.5 45.8 51.0(CH3)3NOK+ 98.8 44.5 43.1

0

50

100

150

200

250

BDE of C-N bond

BDE of N-O bond

(CH3)3NO+· (CH3)3NO (CH3)3NOK+ (CH3)3NONa+ (CH3)3NOLi+ (CH3)3NOH+

Fig. 5. Comparison of the bond dissociation energies of the NAC bond and the NAObond in the different states of trimethylamine oxide (kcal/mol).

X. Zhang / Journal of Molecular Structure 981 (2010) 153–158 157

of bond dissociation energies of the NAC bond and NAO bond inthe different states of trimethylamine oxide, including oxidation,protonation and alkali metal ionization. When the trimethylamineoxide is oxidized, namely in the trimethylamine oxide radical cat-ion, both of the bond dissociation energies of the NAO bond andNAC bond decrease, and especially for the former bond, the BDEvaries from 228.3 to 42.0 kcal/mol. As a result, the loss of an elec-tron from trimethylamine oxide is beneficial to the cleavage of theNAC bond and NAO bond; meanwhile, the dissociation of the NACbond remains prior to that of the NAO bond.

On the other hand, protonation and alkali metal ionization canalso change the bond dissociation energies of the NAC bond andNAO bond in the trimethylamine oxide. However, their influenceon the BDEs of the NAC bond and NAO bond is dramatically oppo-site. The interaction between cations (H+, Li+, Na+ and K+) and tri-methylamine oxide would strengthen the NAC bond, and itsbond dissociation energies follow the order of Me3NO < Me3-

NOK+ < Me3NONa+ < Me3NOLi+ < Me3NOH+. It can be deduced thatwith the reduction of cation radius, the interaction energy betweenthe cation and trimethylamine oxide becomes bigger and bigger(Table 2), and that the impact of the cation upon the NAC bondis on the increase, which leads to the rise in the correspondingbond dissociation energy. In contrast to the above-mentioned,the interaction between cation and trimethylamine oxide wouldweaken the NAO bond and its bond dissociation energies followthe order of Me3NO > Me3NOK+ > Me3NONa+ > Me3NOLi+ > Me3-

NOH+. It should attach great importance to the fact that proton ex-

Fig. 4. Bond dissociation energies of the NAO bond and the NAC bond in the neut

erts the greatest influence on the trimethylamine oxide. In neutraltrimethylamine oxide, trimethylamine oxide radical cation and ad-ducts of alkali metal cations (Li+, Na+ and K+) and trimethylamineoxide, the dissociation of the NAC bond has precedence over thatof the NAO bond, while in the protonated trimethylamine oxide,the dissociation of the NAO bond would be more favorable.

4. Conclusion

We have investigated the dissociation of the NAO bond andNAC bond in the protonated trimethylamine oxide and adductsof alkali metal cations (Li+, Na+ and K+) and trimethylamine oxidebased on MS/MS technique and density functional theory (DFT).

ral trimethylamine oxide and trimethylamine oxide radical cation (kcal/mol).

158 X. Zhang / Journal of Molecular Structure 981 (2010) 153–158

CID experiments only exhibit the dissociation of the NAO bond inMe3NOH+ and the dissociation of the NAC bond in Me3NONa+. DFTcalculations have given a reasonable explanation. Due to theintroduction of these cations (H+, Li+, Na+ and K+), the NAC bondin trimethylamine oxide is strengthened and the order of its BDEsis Me3NO < Me3NOK+ < Me3NONa+ < Me3NOLi+ < Me3NOH+, whilethe NAO bond is weakened and the order of its BDEs is Me3NO >Me3NOK+ > Me3NONa+ > Me3NOLi+ > Me3NOH+. On the other hand,only in Me3NOH+, BDENAO < BDENAC, but in Me3NOX+ (X = Li, Naand K), BDENAC < BDENAO. In addition, it must be pointed out thatin Me3NOK+, the OAK bond is easier to break, as it is weaker thanthe NAC bond. In Me3NOLi+, all chemical bonds are too strong tobreak.

Acknowledgements

The author gratefully acknowledges financial support from thestart-up costs to introduce research personnel of Zhejiang Gong-shang University.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2010.07.047.

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