Atmospheric Chlorine Reaction with N-methyl-2-pyrrolidinone (NMP)

: Density functional theory (DFT) and Complete Basis Scale methods (CBS-QB3, G3B3) are used to investigate the reactivity, the mechanism, structure-reactivity relationship and the kinetics of N-methyl-2-pyrrolidinone (NMP) with Cl atom. To obtain rate constants of the reaction, the RRKM theory is employed at atmospheric pressure and temperature range 273–380 K. This study provides the rate coefficients and detailed H-abstraction mechanism for the reaction of Cl with NMP at high level of theoretical methods. The obtained rate constant ~ 0.92 x 10 −10 to 8.98 × 10 −10 cm 3 molecule −1 s −1 at 298 K agreed with those obtained previously for N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMA). The study shows that the reaction mechanism of Cl with NMP goes favorably through an H-abstraction from N-methyl groups and adjacent CH 2 . The rates constants obtained for the three amides confirm our prediction regarding the structure-reactivity relationship where This study represents a theoretical determination of the rate coefficients and detailed H-abstraction mechanism for the reaction of the Chlorine atom with one cyclic amide (NMP), within the temperature range 273–380 K. This work has been achieved using high level of theory DFT, CBS-QB3 and G3B3 methods considered to be the most efficient and accurate computational methods for the study of this kind of reactions. The reaction of NMP with Cl is exothermic and exergonic taking place in five steps as mentioned in our previous work for DMF and DMA. Kinetic and mechanistic results show that the mechanism of the Cl reaction with NMP goes with small barriers through H-atom transfer from –N-CH 3 group and -CH 2 adjacent to the N-CH3 site within a branching ration slightly in favor of the N-CH3 site. The suggested mechanism is maintained by the results for reactions of DMF and DMA with chlorine [28]. The rate constants determined in this study present a troposphere lifetime of 3 to 10 days which could be seen as affect the chemistry of the troposphere on local and regional scales .

synchronous transit" procedure for the search for the saddle point (QST3). The Hessian matrix of TS has a one negative frequency. The localization of the TS as recommanded in a previous criticism were confirmed with intrinsic reaction coordinates calculations (IRC) [12].
As B3LYP method overestimates H-abstraction barriers and provides unreliable thermochemical values.
Heats of formation for all gaseous species involved in the reaction were calculated based on the procedure defined in the literature [24][25].
G3B3 (or G3//B3LYP), a variant of G3 method, is useful for larger open-shell systems with a very low spin contamination (~1 kcal/ mol for the neutral set of molecule) [26]. Enthalpies of formation at 0 K and 298 K were calculated as defined in our previous work. [26][27][28][29].

Structural calculations
Five stationary points on the profile energy surface (PES) have been localized,with a competition of four H-abstraction pathways, for each path: the reactants,the reactant complex, the transition state, the product complex and the products respectively NMP, HCL, CR1-CR4, TS1-TS4, CP1-CP4 and P1-P4.
For N-methyl-2-pyrrolidinone (NMP), H-abstraction may take place (Figure1) at several sites. The reaction is estimated to process with H-abstraction from the three −CH2 − groups of the pentacycle, or −CH3 adjacent to the Nitrogen. The TS's geometry consist of a near-linear alignment of the Cl atom, the H atom to be abstracted, and the atom to which the H atom is attached in the neutral structure. Four channels have been modeled for NMP +Cl.
The reaction H-abstraction pathways progress via three-steps, namely: (i) the formation of the complex CR1-CR4 from the isolated reactants NMP+Cl, (ii) the formation of the complex CP1-CP4 from the reactant complex through the TS1-TS4 and (iii) the formation of the consistent radical and HCl from the product complex Fig 1−4. Table 1-6 regroup the energies of reactants, complex reactants , Ts's, complex products and products species involved in the mechanism.
The experimental and theoretical enthalpies are in good agreement except for R1-R4 species Fig 1 for which the experimental enthalpies are not available in the literature as mentionned previously in [26][27][28][29].  Table 4.

H-Abstraction from −CH 2 adjacent to the N-CH 3 group
In this path the H-abstraction process includes the hydrogen of the −CH 2 -group adjacent to the N-CH 3 .
The CP2 structure shows an intermolecular H-bond between the carbon of the group -CH 2 -adjacent to the N-CH 3  process is exothermic and exergonic (seeTable 4) as in the previous path .

H-Abstraction from Group −CH 2 central and −CH 2 adjacent to the C=O group
The H-abstraction process in this path includes the hydrogen of the CH 2 -group central in the NMP structure. The products complex involve an intermolecular hydrogen bond between the carbon atom of the For theses path's, although the reactions process are lightly exothermic and exergonicsee (see Table 4) , the calculations shows a very high barrier compared to the other paths and were not taken in consideration in the Kinetic calculations without considering the tunneling effect that may occur in these cases.

KINETICS
The mechanism of H-abstraction reaction of NMP with Cl is studied as a competition between two Habstractions as follows.
as seen previously in the Cl reaction with DMF and DMA, and according to the PES's, the rate constant (k) of the four channels can be analyzed in terms of conventional transition-state theory (CTST) or RRKM/ME calculations. k Ia , k IIa , represent the forward rates, k −Ia , k −IIa k Ib , k IIb the reverse ones for the first step and the second step respectively for channel Ia, while k Ic , k IIc , represent the forward rates and k -Ic , k −IIc the reverse ones for channels Ib.
Rate constants (k I , k II ) of channels Ia−Ib can be obtained according to this hypothesis, as: Where k eqIa , k eqIIa , k eqIc , k eqIIc , represent the equilibrium constants connecting the isolated reactants and the complexes for channel I and channel II respectively.
The rates constants involved in the H-abstraction mechanism were carried out with ChemRate program [14]. Simulations were determined at 1atm and 273-380 K. Collision energy transfer was designated by an exponential-down model, with ΔE down =300 cm -1 , Ar (argon) considered as bath gas with lennard-jones parameters of σ=4.4 and ξ=216 K [38][39]. All rate constants obtained in this study are in s -1 or cm 3 molecule-1 s -1 units.
The k overall (c), the branching ratios (d) obtained from the kinetic simulation model are calculated as The calculations of the transmission coefficients ( , using the Wigner, Skodje, and Truhler formalisms [42][43] were obtained in the range 278−400 K. Small tunneling effect was observed (0.994 to 1.004) saying no effect on the rate constant values calculated in this study as seen in our previous work for DMF and DMA [28].

RESULTS AND DISCUSSION
The H-abstraction from N-methyl -2-pyrrolidinone (NMP) reaction as described above, has been modeled according to a complex mechanism in the entrance channel: two complexes CR1 and CR2 have been considered with the lowest two transition states TS1 and TS2 respectively followed by two complexes CP1 and CP2 for each reaction profile.
The H-abstraction from the -N-CH 3 group site leads to the species PC1. This path was found to be slightly dominant with a branching ratio = 0,55 , as for DMA, toward electrophilic attack due to the positive inductive effect of these groups can be stated for this reaction. Significant negative temperature effect was noted for k I and k II . overall vary from 8.87(E-10) to 1,05(E-09) cm 3 molecule -1 s -1 in the temperature range 273-380 K (see Table 5 The rate constant of the reaction of Cl with NMP decrease as the temperature increases in the range 273-380 K and are in good agreement with the results of the reaction Cl with DMF and DMA [28]. In the literature the reaction of DMA and DMF with CL [8], state that for DMA and DMF, abstraction occur with a branching ratio of 55 %, in N-CH3 groups [28]. This mechanistic tendency is also observed in the theoretical calculations, where the H-abstracted from the -N-CH3 group appeared to be slightly dominant channel with small barrier energy and negative temperature dependence. For the reaction of NMP with Cl over the temperature range explored, an important effect of the temperature is observed.

Conclusion
This study represents a theoretical determination of the rate coefficients and detailed H-abstraction mechanism for the reaction of the Chlorine atom with one cyclic amide (NMP), within the temperature range 273-380 K. This work has been achieved using high level of theory DFT, CBS-QB3 and G3B3 methods considered to be the most efficient and accurate computational methods for the study of this kind of reactions. The reaction of NMP with Cl is exothermic and exergonic taking place in five steps as mentioned in our previous work for DMF and DMA. Kinetic and mechanistic results show that the mechanism of the Cl reaction with NMP goes with small barriers through H-atom transfer from -N-CH 3 group and -CH 2 adjacent to the N-CH3 site within a branching ration slightly in favor of the N-CH3 site.
The suggested mechanism is maintained by the results for reactions of DMF and DMA with chlorine [28].
The rate constants determined in this study present a troposphere lifetime of 3 to 10 days which could be seen as affect the chemistry of the troposphere on local and regional scales.