Results and discussion [610281]

Results and discussion
The DFT results regarding theenthalpy offormation (ΔfH0)at0and298.15K,theGibbs free
energy (ΔfG0)at298.15Kandsome bond lengths aregiven inTables 1and2,while theO4-C4-C5,
O5-C5-C4andO4-C4-C5-O5angle values aregiven inTable 3.
Themolecular models fortheconsidered radical cations (B3LYP/ 3-21G)areshown inFig.5.
The observed position ofthelong bond generally coincides with ourprevious results obtained
using theRM1andPM7semi -empirical methods .
The Annual International Conference of the Romanian Society for Biochemistry &
Molecular Biology, Timișoara, Romania, June 7
–
9, 2017
Long bonds involved in EI
–
MS fragmentation of aldose
derivatives predicted using advanced basis sets
Mihai -Cosmin PASCARIU1,2,3*, Madian RAFAILĂ4, Mihai MEDELEANU4, Ramona CURPAN5,
Nicolae DINCĂ6, Alina GEORGESCU7, Eugen ȘIȘU7*
1“Vasile Goldiș” Western University of Arad, Faculty of Pharmacy, 86 Liviu Rebreanu, RO -310414 Arad, Romania
2National Institute of Research & Development for Electrochemistry and Condensed Matter –INCEMC, 144 Dr. Aurel Păunescu -Podeanu , RO-300569 Timișoara, Romania
3“Chemeia Semper” Association, 6A Giuseppe Verdi, RO -300493 Timișoara, Romania
4University Politehnica Timișoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 Vasile Pârvan Blvd., RO -3002 23 Timișoara, Romania
5Institute of Chemistry Timișoara of Romanian Academy, 24 Mihai Viteazul Blvd., RO -300223 Timișoara, Romania
6“Aurel Vlaicu” University of Arad, Faculty of Food Engineering, Tourism and Environmental Protection, 2 Elena Drăgoi, RO -310330 Arad, Romania
7“Victor Babeș” University of Medicine and Pharmacy of Timișoara, Faculty of Medicine, 2 Eftimie Murgu Sq., RO -300041 Timișoara, Romania
* Corresponding author: [anonimizat] , [anonimizat]
Computational methods frequently predict long bonds inradical cations ofvicinal diols, likesugars .Although semi -empirical methods offer lengths forsuch bonds thatarelikely
tobeexaggerated, themore advanced DFT methods cangive better results .Inthiswork wehave performed DFT computations withlarge basis sets tocalculate thelength oftheC4-
C5bond inradical cations ofacetalized monosaccharides .Theresults correlate wellwithourprevious studies regarding theposition ofthelong bond .Thelengths ofthese bonds are
slightly shorter when compared withthevalues given bysemi -empirical methods, likeRM1andPM7.
Acknowledgement: This work was supported by the Romanian National Authority for Scientific Research (CNCS -UEFISCDI) through projects PN -II-PCCA -2011-142 and POSCCE 185/48749 (contract 677/09.04.2015).
References:
1. D.J. Bellvile, R.A. Pabon, N.L. Bauld, J. Am. Chem. Soc. 1985 , 107, 4978 -4979.
2. M.C. Pascariu, M. Rafailă, M. Medeleanu, V. Badea, A.T. Gruia, V.L. Purcarea, M. Penescu, E. Sisu, Farmacia (Bucharest, Romania) 2016 , 64, 553 -564.
3. M.C. Pascariu, L.A. Bozin, A. Șerb, N. Dincă, E. Șișu, Proceedings of The 21st International Symposium on Analytical and E nvironmental Problems, September 28, 2015 , Szeged, Hungary, 247 -250.Introduction
Long bonds (Fig.1)appear tobeaubiquitous feature inthemolecular radical cations produced
during thepositive mode electron ionization mass spectrometry (EI-MS) analysis ofcompounds
possessing vicinal electron donor substituents, likediols ordiethers [1].Carbohydrates areanexample
ofsuch compounds .While semi -empirical methods [2,3]give long bond lengths forradical cations that
arelikely tobeexaggerated, DFT computations areknown tobemore moderate inthisrespect .To
verify thisassumption wehave performed DFT analyses onradical cations offivemonosaccharide
derivatives, which contain thefuranose moiety (Fig.2):1,2:5,6-di-O-isopropylidene -α-D-glucofuranose
(DAG), 1,2:5,6-di-O-isopropylidene -α-D-allofuranose (DAAlo), 1,2:5,6-di-O-isopropylidene -α-D-
galactofuranose (DAGal), 2,3:5,6-di-O-isopropylidene -α-D-mannofuranose (α-DAM) and 2,3:5,6-di-O-
isopropylidene -β-D-mannofuranose (β-DAM) .Allthese species show, during theEI-MSanalysis, a
preference forthebreaking oftheC4-C5bond, with theformation ofthe2,2-dimethyl -1,3-dioxolan -4-
ylium oxocarbenium ion,which exhibits ahigh intensity m/z=101peak [2,3](Figs.3-5).Materials and methods
Allstructures were initially modeled using theHyperChem 8.0.10software .Thestarting neutral
molecules, obtained after “MM+” pre-optimization, were optimized withtheRM1semi -empirical method .
Theradical cations were obtained from these structures andwere pre-optimized withRM1.Asfor“Spin
Pairing”, RHF operators were used forneutral molecules, while UHF operators were employed for
radical -cations .The SCF “Convergence limit” was set at10-5,without using the “Accelerate
convergence” procedure .Forgeometry optimization, the“Polak -Ribière (conjugate gradient)” algorithm
was selected with aRMS gradient of0.01kcal (Åmol)-1,themolecules being considered invacuum
(conditions similar tothose found inEI-MSdetectors) .
Theoretical calculations were finally performed using theGaussian 09software .Theequilibrium
geometries oftheradical cations were optimized using thedensity functional theory (DFT) method at
theB3LYP/ 3-21GandB3LYP/ 6-311++G( 2d,2p)levels .TheB3LYP hybrid functional was used forthese
studies because ofconvention andthesuccessful usetomodel arange ofgas-phase reactions .
Conclusions
Incontrast withourprevious results [2,3]obtained using theRM1andPM7semi -empirical methods, which give long
bonds ofslightly over 2Å,theDFT methods applied offered long bonds ofmaximum 1.712 Å.The more refined 6-
311++G(2d,2p)basis setgives better results withrespect tothelong bond location, when compared withthemore simple 3-
21Gbasis set.Figure 3 ►.Genesis of the 2,2 -dimethyl -1,3-dioxolan -4-ylium cationFigure 1 ▲.Stabilization of radical cations in vicinal groups containing compounds
Figure 4 ▼.α-Oxy resonance stabilization in 2,2 -dimethyl -1,3-dioxolan -4-ylium ion
Figure 5. EI-MS spectra for (from top to bottom) DAG, DAGal, α -DAM, β -DAM
Figure 2 ◄.Structure of the studied compounds
Table 3. Angle values (degrees) obtained for the studied radical cationsTable 2. Results obtained for the radical cations using the B3LYP/3 -21G level of theoryTable 1. Results obtained for the radical cations using the B3LYP/6 -311++G(2d,2p) level of theory