Study of Cyclic Quaternary Ammonium Bromides by B3LYP Calculations, NMR and FTIR Spectroscopies

N,N-dioctyl-azepanium, -piperidinium and -pyrrolidinium bromides 1-3, have been obtained and characterized by FTIR and NMR spectroscopy. DFT calculations have also been carried out. The optimized bond lengths, bond angles and torsion angles calculated by B3LYP/6-31G(d,p) approach have been presented. Both FTIR and Raman spectra of 1-3 are consistent with the calculated structures in the gas phase. The screening constants for 13C and 1H atoms have been calculated by the GIAO/B3LYP/6-31G(d,p) approach and analyzed. Linear correlations between the experimental 1H and 13C chemical shifts and the computed screening constants confirm the optimized geometry.


Introduction
Quaternary ammonium compounds (QACs) were introduced as antimicrobial agents by Domagk over seventy years ago [1]. The first generation of QACs were standard benzalkonium chlorides, i.e. alkylbenzyldimethylammonium chloride, with specific alkyl distributions, i.e., C 12 , 40%; C 14 , 50% and C 16 , 10% [2]. The second generation of QACs was obtained by substitution of the aromatic ring in alkylbenzyldimethylammonium chlorides by chlorine or alkyl groups to get products like alkyldimethylethylbenzylammonium chloride with C 12 , 50%; C 14 , 30%; C 16 , 17% and C 18 , 3% alkyl distribution. Dual quaternary ammonium salts are the third generation of QACs. These products are a OPEN ACCESS mixture of equal proportions of alkyldimethylbenzylammonium chloride with alkyl distribution C 12 , 68%; C 14 , 32% and alkyldimethylethylbenzylammonium chloride with alkyl distribution C 12 , 50%; C 14 , 30%; C 16 , 17% and C 18 , 3%. The twin chain quaternary ammonium salts, like didecyldimethylammonium chloride are the fourth generation of QACs. The concept of synergistic combinations of dual QACs has been applied to twin chain quaternary ammonium salts. The mixture of dialkyldimethylamoonium chloride (dioctyl, 25%; didecyl, 25%, octyldecyl, 50%) with benzalkonium chloride (C 12 , 40%; C 14 , 50%; C 16 , 10%) is the newest blend of quaternary ammonium salts which represents the fifth generation of QACs [2]. Because of the increasing resistance of microorganisms to commonly used disinfectants, the synthesis of new types of microbiocides is very important. One of the new groups with good antimicrobial activity are the cyclic quaternary ammonium salts [3]. The aim of this work was the synthesis of cyclic N,N-dioctyl quaternary ammonium salts, i.e. N,N-dioctylazepanium, N,N-dioctylpiperidinium and N,N-dioctylpyrrolidinium bromides, with potential antimicrobial activity. Some cyclic quaternary ammonium salts have previously been obtained by intramolecular cyclisation of amine derivatives [4][5][6][7][8][9]. Another way, i.e. reaction of alkyl halides with cyclic amines, can lead to chiral cyclic quaternary ammonium salts [10].
The molecular structures of N,N-dioctyl-azepanium (1), -piperidinium (2) and -pyrrolidinium (3) bromides analyzed by FTIR and NMR spectroscopy and B3LYP calculations are presented in this paper. The above compounds belong to the cyclic quaternary ammonium bromide family investigated in our laboratory in order to better understand the mechanism of their biological activity.

Synthesis
N,N-dioctyl-azepanium, -piperidinium and -pyrrolidinium bromides 1-3 were obtained by reaction of N,N-dioctylamine with dibromohexane, dibromopentane and dibromobutane, respectively. The reaction of secondary amines with 1,5-dichloropentane and 1,4-dichlorobutane to produce dialkylpiperidinium and dialkylpyrrolidinium salts has previously been described by Ericsson and Keps [4]. In our work, using dibromoalkanes instead of dichloroalkanes, we formed five-, six-and seven-membered ammonium compounds in much higher yields and after shorter reaction times. In the first step of reaction of dioctylamine with α,ω-dibromoalkane, the halogenated tertiary amine is formed, which shows a strong tendency to form cyclic quaternary ammonium salts.

B3LYP Calculations
The structures and numbering for 1-3 are given in Figure 1. The structures optimized at the B3LYP/6-31G(d,p) level of theory are shown in Figure 2.   The computed B3LYP geometry parameters, energy and dipole moments are given in Table 1. The calculated energy for N,N-dioctylazepanium bromide (1) is about 1.2% lower than for N,Ndioctyl-piperidinium bromide (2) and 2.4% lower in comparison to N,N-dioctylpyrrolidinium bromide (3). The bromide anions in 1-3 are engaged in three non-linear weak intramolecular interactions with carbon atoms. Bromide anions additionally interact via Coulombic attractions with positively charged nitrogen atom. The N + (…)···Brdistances are 3.888 Å, 3.709 Å 3.674 Å, for 1, 2 and 3, respectively.

FTIR and Raman Spectra Study
Room-temperature solid-state FTIR and Raman spectra as well as the calculated spectra of 1 are shown in Figure 3.  The observed and calculated harmonic frequencies and their tentative assignments are listed in Table 2. In general, the calculated frequency values with B3LYP 6-31G(d,p) basis set are close to experimental values of vibrational frequency.  The abbreviations used are: s, strong; m, medium; w, weak; vw, very weak; ν, stretching; β, in plane bending; δ, deformation; γ, out of plane bending; and τ, twisting.

1 H-NMR and 13 C-NMR Spectra
The proton chemical shift assignments (Tables 3-5) are based on 2D COSY experiments, in which the proton-proton connectivity is observed through the off-diagonal peaks in the counter plot. The relations between the experimental 1 H and 13 C chemical shifts (δ exp ) and the GIAO (Gauge-Independent Atomic Orbitals) isotropic magnetic shielding (σ calc ) for 1 is shown in Figure 4. Both correlations are linear, described by the relationship: δ exp = a + b·σ calc . The parameters a and b are given in Tables 3-5. The very good correlation coefficients (r 2 =0.9379) for 1 H and (r 2 =0.9984) for 13 C confirm the optimized geometry of 1-3.      The correlation between the experimental chemical shifts and calculated isotropic screening constants are better for 13 C atoms than for protons. The protons are located on the periphery of the molecule and thus are supposed to be more efficient in intermolecular (solute-solvent) effects than carbons. The differences between the exact values of the calculated and experimental shifts for protons are probably due to the fact that the shifts are calculated for single molecules in gas phase. For this reason the agreement between the experimental and the calculated data for proton is worse than for 13 C.

Conclusions
N,N-dioctyl-azepanium, -piperidinium, -pyrrolidinium bromides 1-3 have been obtained by reaction of N,N-dioctylamine with dibromohexane, dibromopentane and dibromobutane, respectively. The structure of the investigated compounds has been analyzed by FTIR and NMR spectroscopy and B3LYP calculations. Both the FTIR and Raman spectra of 1-3 are consistent with the observed structures in the gas phase. The good correlations between the experimental 13 C and 1 H chemical shifts in D 2 O solution and GIAO/B3LYP/6-31G(d,p) calculated isotropic shielding tensors (δ exp = a + b·σ calc ) have confirmed the optimized geometry of 1-3.

General
The NMR spectra were measured with a Varian Gemini 300VT spectrometer, operating at 300.07 and 75.4614 MHz for 1 H and 13 C, respectively. Typical conditions for the proton spectra were: pulse width 32 o , acquisition time 5s, FT size 32 K and digital resolution 0.3 Hz per point, and for the carbon spectra pulse width 60 o , FT size 60 K and digital resolution 0.6 Hz per point, the number of scans varied from 1200 to 10,000 per spectrum. The 13 C and 1 H chemical shifts were measured in CDCl 3 relative to an internal standard of TMS. All proton and carbon-13 resonances were assigned by 1 H (COSY) and 13 C (HETCOR). All 2D NMR spectra were recorded at 298 K on a Bruker Avance DRX 600 spectrometer operating at the frequencies 600.315 MHz ( 1 H) and 150.963 MHz ( 13 C), and equipped with a 5 mm triple-resonance inverse probehead [ 1 H/ 31 P/BB] with a self-shielded z gradient coil (90 o 1 H pulse width 9.0 μs and 13 C pulse width 13.3 μs). Infrared spectra were recorded in the KBr pellets using a FT-IR Bruker IFS 66 spectrometer. The Raman spectrum was recorded on a Bruker IFS 66 spectrometer. The ESI (electron spray ionization) mass spectra were recorded on a Waters/Micromass (Manchester, UK) ZQ mass spectrometer equipped with a Harvard Apparatus syringe pump. The sample solutions were prepared in methanol at the concentration of approximately 10 -5 M. The standard ESI -MS mass spectra were recorded at the cone voltage 30V.

General procedure for the synthesis of N,N-dioctylcycloalkylammonium salts 1-3
Dioctylamine (5 g, 0.02 mol) was mixed with the appropriate dibromoalkane (0.02 mol) in the presence of anhydrous sodium carbonate (4.14 g, 0.04mol). The reaction mixture was heated under reflux for 15 h. The solvent was evaporated under reduced pressure and the residue was dried over P 4 O 10 and then recrystallized from a suitable solvent, as indicated.