Chlorine Nuclei Research Articles

Radical cations of ethylene oxide and other oxiranes prepared by exposure to ionizing radiation. An electron spin resonance study

Exposure of dilute solutions of tetramethyl oxirane in trichlorofluoromethane to 60C γ-rays at 77 K gave a species identified as the ring-opened radical cation, [graphic omitted]. Its e.s.r. spectrum comprised 13 lines with A(1H)= 15.1 G. This species had an intense violet colour, and there was no evidence for the cyclic cation. Similarly, the cations of cyclopentene oxide and cyclohexene oxide gave red–violet colours and complex e.s.r. spectra which were interpreted in terms of the oxirane ring opening to give 6- and 7-membered cyclic cations only.Both the 2,3-cis-dimethyl and the monomethyl derivatives on exposure at 4 K gave poorly defined e.s.r. spectra and no coloration. On warming to ca. 30 K the spectra became better defined and at 77 K intense red colours appeared together with new e.s.r. features which could be analysed in terms of the open-ring structures.Oxirane itself gave a spectrum with splitting due to four equivalent protons, together with small extra splittings probably due to coupling to a chlorine nucleus. The proton coupling of ca. 16 G is close to that for the two α-protons of the open-chain radical (Ċtext-decoration:overlineHtext-decoration:overlineMtext-decoration:overlineetext-decoration:overlineOĊHMe)+, which strongly supports an open structure. However, there was no visible absorption band under all conditions in CFCl3. In marked contrast, a very similar e.s.r. spectrum of five lines was obtained from solutions in SF6 at 77 K, but these solutions had an intense orange–red coloration. These results are tentatively interpreted in terms of a cyclic cation for oxirange in CFCl3, but an ‘open’ cation in SF6, both having very similar proton hyperfine coupling constants. Similarly, the acquisition of colour and change in the e.s.r. spectrum of the monomethyl and dimethyl derivatives is explained in terms of ring-opening in the 40–70 K region for solutions in CFCl3.

The microwave spectrum of the PCl radical in the X 3Σ− state

The microwave spectrum of the P35Cl radical in the ground state has been observed by using a source modulation microwave spectrometer. The PCl radical was produced in a 3.5 m long free space cell by a dc glow discharge in the PCl3 gas. The seven rotational transitions of N=5←4 up to 11←10 were observed in the frequency region of 75 to 169 GHz. The observed spectral lines, 105 in total, were analyzed to give the molecular constants: B0=7558.3464(13), D0=0.006 372 1(45), γ0=−91.525(30), λ0=127 542.92(81), and λD=0.136 76(66), all in MHz with three standard errors in parentheses. The magnetic hyperfine coupling constants have also been determined for both the phosphorus and chlorine nuclei. By comparing these constants with the atomic values the spin density and the s character of the unpaired electron orbital have been calculated to be 87.2% and 0.86% for P and 14.1% and 0.11% for Cl. The Cl nuclear electric quadrupole coupling constant has been determined to be −44.6(15) MHz with the three standard deviation in parentheses.

1H NMR Studies on the Motion of Guanidinium Ions and Dipolar‐Quadrupolar Cross Relaxations in Crystalline Guanidinium Tetrachloro‐ and Tetrabromometallates Including Au(III), Pt(II), or Pd(II) as Central Metal Ions

AbstractThe temperature dependences of proton spin‐lattice relaxation times T1 at various Larmor frequencies and of 1H NMR second moments at 40 MHz were determined for (guH)AuCl4, (guH)AuBr4, (guH)2PtCl4, (guH)2PtBr4, (guH)2PdCl4, and (guH)2PdBr4, where guH indicates a guanidinium cation [C(NH2)3]+. A solid‐solid phase transition between stable phases was located for (guH)AuCl4, (guH)AuBr4, and (guH)2PtBr4 at 363, 387, and 359 K, respectively. The high‐temperature phases of the three complexes were easily supercooled. For (guH)AuBr4, another phase transition between substable phases was found at 218 K. From the measurements of 1H T1, the motional parameters for the C3 reorientation of the D3h cation in the all complexes studied were evaluated. Below room temperature, (guH)2PtCl4 and (guH)2PdCl4 yielded unusual temperature dependences of 1H T1 unexplainable by the usual BPP theory. Moreover, the 1H T1 values in this temperature region depended strongly and in an anomalous manner on the 1H Larmor frequency employed. These unusual temperature and frequency dependences of 1H T1 can be explained as that the 1H T1 values are drastically affected by the temperature‐variable quadrupolar relaxation of chlorine nuclei in the complex anions through dipolar‐quadrupolar cross relaxation between 1H and chlorine nuclei. The frequency dependence of 1H T1 of (guH)2PtCl4 and (guH)2PdCl4 determined at various temperatures was analyzed by the Kimmich method and it revealed that the onset of anionic motion, presumably the 90° reorientation of the anion approximately as a whole about its C4 axis occurs near 200 and 250 K, respectively, with a rather small activation energy of ca. 30 kJ mol−1 for both complexes. The correlation time of chlorine nuclei for the C4 reorientation of the anions obtained at various temperatures indicated that the fade‐out phenomenon of 35Cl NQR signals observed for these complexes can be explained by the life‐time broadening. From the frequency dependence of 1H T1, the quadrupole coupling constant of chlorine nuclei for both complexes could be roughly estimated at higher temperatures where no signals could be detected by the usual NQR technique because of the fade‐out phenomenon. For (guH)AuCl4, unusual temperature and frequency dependences of 1H T1 attributable to the cross relaxation between 1H and chlorine nuclei were also observed above ca. 300 K and below ca. 230 K.

An alternative method of determining the quadrupole moments of the chlorine nuclides

From the hyperfine structure of the transition Cl II λ = 6 663.56 A : 3p3(2D)3d 3G5 - 3p3( 2D)4p 3F4 the hyperfine constants of the participating levels could be deduced. In particular, the B( 3F4 )-value is identical with the b-value of the 4p-electron. To derive novel values of the quadrupole moments of the chlorine nuclei A = 35 and 37 the expectation value 4p is needed. We infer it from A(3F 4) as well as from the spin orbit constant ζ4p to sufficient agreement We hence obtain Q(35Cl) = - 8.2 ± 0.4 x 10 -30 m2 which must be seen in contrast to Korolkov and Makhanek's value — 8.5 x 10-30 m2. The latter value as any other available value is taken from the state Cl I 3p5 2 P3/2. Our value is the first one not taken from the ground configuration of Cl I. It should be less sensitive to Sternheimer polarizations and hence be suited for a proof of calculated Sternheimer corrections.

Non-empirical calculations of chlorine quadrupole coupling parameters in chloromethanes and chlorosilanes

Non-empirical calculations of the field gradient tensor at the chlorine nucleus are resorted for chloromethane, trichloromethane, chlorosilane and trichlorosilane. The calculations agree quite well with the experimental quadrupole coupling constants. The calculated asymmetry parameters in trichloromethane and trichlorosilane are low and the principal z axis of the field gradient tensor is found nearly to coincide with the CCl and SiCl bond directions.

Radiation chemical formation of radical cations of halogenoalkanes and their electron spin resonance spectra and structure

Exposure of dilute solutions of chloro-, bromo- and iodo-alkanes in trichlorofluoromethane to 60Co γ-rays at 77 K gave the corresponding cations. However, for RCI+ and RBr+ the hole is shared with one chlorine atom of a solvent molecule via the formation of a σ bond, as in [RCI[graphic ommitted]CICFCI2]+. For alkyl chlorides the hyperfine coupling constants of the two chlorine nuclei are nearly equal. The (RBr)+ adduct shows e.s.r. spectra consisting of dominant quartet splittings from bromine and additional quartet splittings from chlorine. For the iodo-cations, no clear evidence for such bonding is obtained, and there are very large shifts in the g-values resulting from marked spin-orbit coupling. For more concentrated solutions, the e.s.r. spectra show septet hyperfine patterns arising from coupling to two equivalent halogen nuclei, which indicates the formation of dimer radical cations, (RX)2+, again having the unpaired electron in a σ* orbital. In the particular case of t-butyl chloride, neither the parent cation, nor its complex with a solvent molecule were detected. Instead, a multiline spectrum was obtained which we assign to (Me2CCH2)+ cations weakly complexed with HCl. Theoretical calculations on the parent cations, their dimers, and adducts with CFCl3 support these assignments.

Spin delocalization in radical cations of oxygen-containing organic compounds as revealed by long-range hyperfine interactions and solvent effects

E.s.r. evidence is presented for long-range 1H hyperfine interactions and thermally reversible solvent effects involving mainly the radical cations of aldehydes and ketones in Freon matrices. Long-range couplings to δ protons are found to be surprisingly large in the cations of acyclic, cyclic and polycyclic carbonyl compounds. The conformational requirements for these large couplings are discussed in terms of facile spin transmission via a trans arrangement of C—C and C—H σ bonds, the assumption of this trans effect providing a rigorous and consistent analysis of the experimental results for both non-rigid and rigid molecular geometries. The matrix perturbation observed at low temperatures is believed to arise from a weak complex formed between the radical cation and the solvent, the hyperfine interaction occurring mainly with a single chlorine nucleus. The reversible loss of the e.s.r. substructure associated with this complex at elevated temperatures is attributed to motional averaging of the chlorine hyperfine anisotropy rather than to dissociation of the complex.

Radical cations of benzyl chlorides and bromides formed by radiolysis. An electron spin resonance study

Exposure of dilute solutions of benzyl chloride, benzyl bromide and various derivatives thereof in trichlorofluoromethane to 60Co γ-rays at 77 K gave the radical cations, characterised by their e.s.r. spectra. The major species obtained from C6H5CH2Cl had hyperfine coupling to three inequivalent protons; assigned to the para ring proton (12 G) and the two methylene protons (26 and 21.5 G), with no detectable coupling to the chlorine nucleus. Only a single isotropic feature was obtained from C6D5CD2Cl, confirming the absence of significant coupling to chlorine. It is suggested that the preferred conformation has the chlorine atom close to, but not in, the plane of the benzene ring, with a significant barrier for the in-plane site.In contrast, only cations with strong hyperfine coupling to bromide nuclei were obtained from benzyl bromide. The p-methyl derivative gave only one structure [A∥(81Br)= 162 G] but benzyl bromide gave two major species [A∥(81Br)= 193 and 133 G] thought to be two alternative conformations of the parent cation. All these cations were sensitive to visible light, but we were unable to detect any well defined radical photoproducts.Reasons for the formation of different conformations are discussed and the results are compared with optical studies for matrix-isolated cations and with e.s.r. results for β-chloro-and β-bromo-alkyl radicals.

Stark effect in the J = 3 –4 microwave line of tert-butyl chloride molecule

Stark effect in the J = 3 – 4 transition spectrum of the tert-butyl chloride has been investigated. The perturbation of the rotational levels by combined effect of the quadrupole and an intermediate electric fields was calculated and used for identify the Stark components. The rotational spectrum of tert-butyl chloride /CH 3/ 3CCl is characterised by the rotational constants A = 4518.70 MHz[1] and B = 3017.85 MHz, dipole moment U= 2.12 D, and quadrupole interaction constant eqQ = -67.58 MHz [2,3]. The energy of rotational states of the tert-butyl chloride molecule is described by the Hamiltonian ▪ where H 0 is the unperturbed rotational energy, H Q that of interaction of the angular momentum and quadrupole moment of the chlorine nucleus, and H E the Stark energy. Since H Q ∼ H E for the electric field strengths applied during the present investigation, we are dealing with the case of intermediate fields. To solve the problem theoretically we had recourse to the representation in F-space and calculated the energy for the perturbation H Q + H E as done by Low and Townes [4]. The theoretical distances of the Stark components from the absorption line of tert-butyl chloride for various electric field strengths at J = 3 − 4 and ΔM F = 0 is shown in Fig. 1a and 1b. We applied the theoretically calculated distances of the Stark components from the absorption line of /CH 3/ 3CCl as function of the electric field strength as well as the relative intensities of the components in order to interpret our experimental results concerning the Stark effect.

35Cl NQR and 1H NMR studies of [C(NH 2) 3]AuCl 4, [C(NH 2) 3] 2PtCl 4, and [C(NH 2) 3] 2PdCl 4

The temperature dependence measurements of 35Cl NQR frequencies and 1H NMR spin-lattice relaxation time T 1 were carried out for guanidinium tetrachloro-aurate(III), -platinate(II), and -palladate(II). The gold(III) complex showed four NQR lines at various temperatures between 77 and 344 K, while the platinum-(II) and palladium(II) complexes gave two NQR lines in the temperature ranges 77–169 K and 77–220 K, respectively. An unusual phase transition was located at 363 K for the gold(III) complex. The high-temperature phase was easily supercooled. All the complexes studied yielded a T 1 minimum attributable to the reorientation of the planar cation about its C 3 axis. The motional parameters were evaluated. The Zeeman-quadrupole cross relaxation between protons and chlorine nuclei was observed for the platinum(II) and palladium(II) complexes at various temperatures below room temperature, while it was also detected for the high-temperature phase of the gold(III) complex.

Radical cations of aldehydes and ketones

We confirm the observation made by Snow and Williams in the previous letter that the quartet substructure observed in the ESR spectrum of acetaldehyde is due to hyperfine coupling to a single chlorine nucleus of the solvent, fluorotrichloromethane. Reasons for this reversible interaction, and its absence for other similar cations are discussed.

The structure of BCl2: ESR study of an x-irradiated crystal of BCl3

X irradiation of a single crystal of boron trichloride at 77 K yields at least two different radicals. A brief exposure to UV irradiation, however, leaves only one radical whose ESR spectrum shows hyperfine coupling from one boron (10B, 11B) and two equivalent chlorine (35Cl, 37Cl) nuclei and which is identified as BCl2. The g tensor, 11B magnetic hyperfine tensor, 35Cl magnetic and quadrupole hyperfine tensors have been determined and analyzed in terms of the radical’s electronic structure. Problems arising in relating the principal direction of the various tensors to the molecular geometry are discussed. BCl2 is nonlinear with a ClBCl angle of approximately 122° and with the unpaired electron mainly localized on the boron atom.

Chlorine-35 qttadrupole relaxation and molecular motion in (Cl 3PNR) 2 crystals

The temperature dependence of the cnlorine-35 nuclear quadrupole spin-lattice relaxation time, obtained for the (CI 3PNCH 3) 2 crystal, shows the existance of a thermal-activated process with the energy barrier 25 kcal/mole. This process seems to be connected with the intramolecular rearrangement of the chlorine nuclei, the most probable mechanism being a trigonal twist which is one of several kinds of ligand exchange taking place in pentacoordinated phosphorus molecules. The effects of such a motion in NCR are discussed.

N. M. R. Investigations of Phase Transitions in Pure and Mixed Crystals of Pentachlorophenol

Abstract The nuclear magnetic resonance line width, second moment and the relaxation times T 1 and T 1D have been measured for protons in a pentachlorophenol (PClPh)-hexachlorobenzene (HClB) mixed crystal (70% in concentration of PClPh), from 120 K to the melting point. The results have been interpreted in terms of the various molecular motions occurring in this crystal. We first observed, as the temperature increased, the fast jump of the proton around the C–O bond, even at low temperature. The activation enthalpy for this motion is 7.5 ± 0.2 KJ mole−1 and its correlation time of ≃ 2.3.10−8 s at ambient temperature. Before the transition (410 K), a slow motion occurs, molecular self diffusion or tumbling, which exhibits a discontinuity in its correlation time at the transition. The hindering enthalpy for this process is found to be 100 ± 20 KJ mole−1 above the transition. The associated correlation frequency is estimated from T 1d to be 50 s−1 near the melting point. These resultscan be extended to other mixed PCIPh-HCIB crystals, for which we have made T I and line-width measurements. Pure pentachlorophenol presents a different picture with two transitions. Below the first (355 K), the structure is rigid, the molecules being linked by hydrogen bonds, in agreement with the X-ray results of Sakurai. Above it, its behavior is similar to that observed for the mixed crystals, up to the high transition (435 K). Above this tem erature up to the me1tingpoint.a diffusive phase evident; the diffusion coefficient being 2 10−3 cm2/s at 460 K. At low temperatures, the dipolar relaxation time is mainly due to dipolar “second kind” re1axation with chlorine nuclei which are strongly relaxed by quadrupolar interactions.

The rotational spectrum, structure, and molecular properties of the ethylene–HCl dimer

Three isotopic species of a hydrogen bonded dimer formed between ethylene and HCl have been observed through assignment of their rotational spectra using a pulsed, Fourier transform microwave spectrometer in which a gas mixture is pulsed into an evacuated Fabry–Perot cavity. The spectra unambiguously reveal a nonplanar, near prolate, asymmetric top (κ = −0.9880) in its ground vibrational state. The equilibrium structure has C2V symmetry with the HCl on the C2 symmetry axis of the dimer, perpendicular to the carbon–carbon bonding axis with the hydrogen atom located between the chlorine and the ethylene molecule, pointing midway between the carbon atoms. The chlorine atom is situated, on average, 3.724 Å from the edge of the ethylene molecule. The following spectroscopic constants have been obtained for the ethylene–H35Cl dimer: the rotational constants are A = 25 457(349), B = 2308.143(3), and C = 2167.970(3) in units of MHz; the chlorine nucleus quadrupole coupling constants are χaa = −54.076(4), χbb = 27.091(6), and χcc = 26.985(10) in units of MHz, and the centrifugal distortion constants are DJ = 7.2(2) and DJK = 282(5) in units of kHz.

Quantum-mechanical calculations of the electric-field gradient at the chlorine nucleus in some small polyatomic molecules

Quantum-mechanical calculations of the electric-field gradient at the chlorine nucleus in some small polyatomic molecules

Bromine nuclear quadrupole resonance studies of some bromocyclotriphos-phazatrienes: N 3P 3Br 6, N 3P 3Br 5NHPr i and [formula omitted

Bromine NQR frequencies, v(MHz), for N 3P 3Br 6, N 3P 3Br 5NHPr i and for cis -N 3P 3Br 4(NMe 2) 2 have been examined over the temperature range 77⩽=T⩽=300K. Bromine-81 resonance frequencies in these compounds are approximately linearly related Co the chlorine-35 resonance frequencies observed in the corresponding chloro-derivatives: v( 81Br) ≈ 8.121 v( 35Cl) - 29.4. Bromine nuclei are more sensitive monitors of physical and chemical change than chlorine nuclei. In the cyclotriphosphazatrienes, bromine is more electronegative than amino-nitrogen. Asymmetry parameters for N 3P 3Br 6 show that π-character cannot contribute any more than about 0.2% to the nature of the P-Br bonds in this molecule. One phase change, at 125K, has been detected for N 3P 3Br 6 and two, at 135K and 215K, for N 3P 3Br 5NHPr i. In cis -N 3P 3Br 4(NMe 2) 2 bromine-81 spectra fade out at 215K. Bromine NQR spectra for N 3P 3Br 5NMe 2 have not been detected in the temperature range 77⩽=T⩽=300K. The NQR data enable conformations to be assigned to these molecules.

Structure and fluxional behaviour of the adducts of orthoquinones with organotin halides. An electron spin resonance study

Free radicals observed in reactions of acenaphthoquinone (AQ) and benzo[2,1-b;3,4-b′]dithiophen-4,5-dione (TQ) with the series of compounds Sn PhnX4–n(n= 3, XH, Cl;n= 2, XCl,n= 0, XCl) have been characterised by e.s.r. spectroscopy. Coupling to only a single chlorine nucleus is detectable in radical adducts containing two or three chlorine atoms. The implications of this observation are discussed in terms of structures involving a five-coordinate tin atom. Activation parameters are reported for the intramolecular migration of SiPh3 and GePh3 groups between the oxygen atoms of TQ.

NMR Spectra of Alkali and Halogen Nuclei in Alkali and Halogen Salts

NMR signals of 7Li, 23Na, 35Cl, 39K, 79Br, 87Rb and 127I have been measured in various alkali and halogen salt powders relative to well defined aqueous solutions. With the known shielding constants of some of these solutions the nuclear magnetic shielding constants of the alkali and chlorine nuclei in crystalline powders were evaluated in the atomic reference scale. The theoretical values of the shielding constants in alkali halides do not agree even in the order of magnitude with the experimental ones in some cases. For 23Na first-order and second-order quadrupole patterns have been observed and the quadrupole coupling constants are given.

Scattering of slow neutrons by the isotopes of chlorine

The Christiansen-filter technique was used to measure the coherent scattering lengthsb for the bound atoms of the chlorine isotopes35C1 and37Cl. The results areb(Cl-35)=11.70 ±0.09 fm/atom andb(Cl-37)=3.08±0.06 fm/atom. Furthermore, the total cross section of natural chlorine for neutron energies 1.2 eV and 5.2 eV was determined by means of transmission measurements with rotating resonance detectors. From these data the spin state scattering lengthsa +(J=2) anda −(J=1) were obtained for the free nuclei of the isotopes. The derived values of the spin- and isotopically incoherent scattering cross section and the total scattering cross section at “zero energy” are well consistent with previously published resonance data. The present results combined with the resonance parameters make possible a complete description of the interaction of slow neutrons with the chlorine nuclei.