Probing very long-lived excited electronic states of molecular cations by mass spectrometry Prof. Myung Soo Kim School of chemistry and National Creative Research Initiative for Control of Reaction Dynamics, Seoul National University, Seoul 151-742, Korea I. Introduction A. Excited electronic states Involved in various processes such as photochemistry, operation of lasers, etc. Difficult to probe. Information scarce. A frontier in physical chemistry research For example, accurate and efficient calculation of excited state energy is the main focus in quantum chemistry. Our interest Utilization of excited electronic states for reaction control B. Fate of an isolated polyatomic system prepared in an excited electronic state 1. Nonradiative decay Internal conversion / intersystem crossing convert the electronic energy into vibrational energy in the ground electronic state. 2. Direct photodissociation on a repulsive state Utilized in our previous work on reaction control via conformation selection (Nature 415, 306 (2002)). 3. Radiative decay – fluorescence Occurs when nonradiative decay is not efficient and electric dipole – allowed transition is present. C. Excited electronic states of molecular ions LUMO HOMO Hole states LUMO states Electron ionization (EI) and VUV photoionization (PI) generate hole states mostly. Peaks in photoelectron spectrum hole states. There are more excited electronic states near the ground state of a molecular ion than that of a neutral ( presence of hole states). Rapid internal conversion prevalent. Fluorescence hardly observed for polyatomic molecular cations. D. Theory of mass spectra 1. Quasi-equilibrium theory (QET) 1) Molecular ions in various electronic (and vibrational) states are produced by EI (or PI). 2) Ions in excited electronic states undergo rapid internal conversion to the ground state. Rapid conversion of electronic energy to vibrational energy. 3) Intramolecular vibrational redistribution (IVR) occurs rapidly also. Transition state theory, or, Rice-Ramsperger-Kassel-Marcus (RRKM) theory. QET or RRKM – QET Wi‡(E - E 0i ) ki (E) i hρ (E) 2. Test Prepare M+ with different E. Measure k i or product branching ratios vs. E. Compare with the calculated results. 3. Results RRKM-QET adequate for most of the cases studied. Some exceptions observed. : Mostly direct dissociation in repulsive excited states. In several cases, dissociation in excited states which do not undergo rapid internal conversion to the ground state suggested. ‘Isolated electronic state’ II. Initial discovery A. Photodissociation of benzene cation Chopper Argon ion laser Prism Laser beam Lens Electric J. Chem. Phys. 113, 9532 (2000) sector Ion beam Electrode assembly Phase- sensitive Schematic diagram of the double Magnetic sector detection focusing mass spectrometer with Laser beam reversed geometry (VG ZAB-E) modified R1 R2R3R4R5R6R7 for photodissociation study. The inset Ion source Collision cell shows the details of the electrode assembly. Ion beam Observed C6H6+ C6H5+, C6H4+, C4H3+, C3H3+ at 514.5nm (2.41eV), 488.0nm (2.54eV), 357nm (3.47eV) Instrument can detect PD occurring within ~1 sec. Electronic states Dissociation ( C6H6+• ) ( Products ) E(eV) 6 ~2 E B2u ktot ~ 107s-1 5 ~ 2 D E1u ktot ~ 104s-1 Energy diagram of the benzene 4 molecular ion. The lowest reaction C6H6+• C6H5+ + H• threshold (E0) is 3.66 eV for 3 ~ 2 C6H6C6H5H. ktot denotes the C A2u total dissociation rate constant in ~2 B E2g the ground state calculated from 2 previous results. 1 ~2 0 X E1g (ground state) For PD to be observed with the present apparatus, photoexcited C6H6+ must have E > 5 eV Remainder ? Photon energy = 2.4 ~ 3.7 eV. A Intensity B 5300 5500 5700 5900 Translational energy, eV PD-MIKE profile for the production of C4H4 from the benzene ion at 357nm obtained with 2.1kV applied on the electrode assembly. Experimental result is shown as filled circles. Reproduction of the profile using the rate constant distribution centered at 6.3107 s-1 obtained by experimental data is shown as the solid curve. The positions marked A and B are the kinetic energies of products generated at the position of photoexcitation and after exiting the ground electrode, respectively. 10 357 nm PD 8 488.0 nm The total RRKM dissociation rate constant of PD BZ as a function of the internal energy -1 calculated with molecular parameters in ref. 8. log k, k in s 6 The internal energies corresponding to the dissociation rate constants of (5.51.1)107 and (53)106 s-1 for PDs at 357 and 488.0 nm, 4 respectively, are marked. 2 4 5 6 7 8 Internal energy, eV Excellent RRKM – QET fitting of k is known for C6H6+ dissociation. From measured k E PD at 357nm (3.47eV) E=6.1 ± 0.1eV Initial E = 2.6 ± 0.1eV PD at 488nm (2.54eV) E=5.5 ± 0.1eV Initial E = 3.0 ± 0.1eV Origin of internal E prior to photoexcitation Most likely vibrational energy acquired at the time of EI, either directly or via internal conversion from an excited electronic state. 2.6 0.1 eV for 357nm PD vs. 3.0 0.1 eV for 488nm PD ? Experimental error? Can we quench it by increasing benzene pressure in the ion source, by resonant charge exchange ? C 6 H 6 +* + C 6 H 6 C 6 H 6 * + C 6 H 6 + PD as a function of C6H6 pressure in the ion source 0.04 Torr Pressure dependences of the precursor (BZ) intensity (–––) and photoproduct (C4H4) intensities at 357 (·····) and 488.0 (---) nm. Relative intensity 0.013 Torr Pressure in the CI source was varied continuously to obtain these data. Pressure was read by an ionization gauge located below the source. The inside source pressures estimated 0.09 Torr at three ionization gauge readings are marked. The scale for the precursor intensity is different 0 from that for photoproduct intensities. -6 -5 -4 10 10 10 Ionization gauge reading, Torr Pig/Torr P/Torr Zc/s-1 tR/s Ncoll 410-6 0.0051 0.13 4.2 0.6 Ion source pressure (P), collision 110-5 0.013 0.33 5.8 1.9 frequency (Zc), source residence time (tR), 210-5 0.025 0.63 7.6 4.8 and number of collisions (Ncoll) suffered by ions exiting the ion source at some 310-5 0.038 0.96 9.0 8.6 benzene pressures. 510-5 0.063 1.59 11.2 18 710-5 0.088 2.23 13.0 29 Quenching mechanism PD at 488nm efficiently quenched (by every collision) resonant charge exchange likely. PD at 357nm hardly quenched. Why? If C6H6+ undergoing PD at 357nm is in an excited electronic state, C6H6 +†+ C6H6 C6H6 + C6H6+† Population of C6H6 +† does not decrease by charge exchange. Charge exchange ionization by benzene cation in the ion source One of the ionization scheme classified as chemical ionization (CI), a useful ionization technique in mass spectrometry. Add small amount of sample (s) to reagent (R) Electron ionization Initially, R+ formed mostly. Charge exchange ionization of S by R+ R+ + S R + S+, electron transfer Translational & vibrational energies are not important to drive this reaction E IE (S) IE (R) Occurs efficiently when E 0, exoergic reactions. Relative intensity of S+ formed by charge exchange with C6H6+ At low C6H6 pressure in the source PD at 357nm occurs Possible presence of long-lived C6H6+, C6H6+†. At high C6H6 pressure complete quenching of PD at 357nm absence of C6H6+†. Samples IE (eV) Low pressure High pressure Chlorobenzene 9.06 3.6 3.5 Fluorobenzene 9.20 3.9 1.4 Benzonitrile 9.62 5.3 0.06 Chloropentafluorobenzene 9.72 4.7 0.01 Nitrobenzene 9.86 3.8 0.06 Hexafluorobenzene 9.91 2.5 0.02 Ionization Energies and the ratios of Ethylene 10.51 3.0 0.02 molecular ion intensities generated by Methylene chloride 11.32 4.4 0.04 charge exchange ionization (CI) with Chloroform 11.37 4.7 0.03 BZ and by electron ionization (EI). Carbon tetrachloride 11.47 3.4 0.06 Ethane 11.52 0.09 ~0 Dichlorofluoromethane 11.75 0.05 0.04 1-chloro-1,1-difluoroethane 11.98 0.16 0.01 Chlorodifluoromethane 12.20 0.09 0.05 Methane 12.51 0.24 ~0 6 9.243 eV 11.5 eV 4 CI/EI ratio 2 0 9 10 11 12 13 Ionization energy, eV C6H6+ generated at high P, fully quenched ionizes samples with IE < 9.2eV. cf. IE (C6H6) = 9.243eV C6H6+ generated at low P ionizes samples with IE < 11.5 eV. ~ cf. IE of C6H6 to A2 E2gstate of C6H6 = 11.488 eV B. Summary Low-lying excited electronic states of C6H6+ ~ ~ ~ X 2 E1g A2 E 2g B2 A2u IE = 9.243 eV IE =11.488 eV IE = 12.3 eV ~ A2 E2g has a very long lifetime, ‘isolated state’. ~2 ~ A E2g X2 E1g electric dipole – forbidden. Internal conversion must be inefficient also. ~ For states above A2 E2g ,internal conversion efficient. (Evidence – failure to ionize S with IE > 11.5 eV by charge exchange) C6H6 Photoelectron Spectrum ~ X ~ A ~ ~ Sharp vibrational peaks for X2 E1g and A2 E2g . III. Charge exchange ionization to detect M+† J.Am. Soc. Mass Spectrom. 12, 1120 (2001). 1. Energetics A+ + B A + B+, E , energy defect E IE (B) RE (A ) For A+ in the ground state, E IE (B) IE (A) E > 0, endoergic = 0, resonant < 0, exoergic 2. Charge exchange cross section 1) Charge exchange between atomic species Massey’s adiabatic maximum rule Maximum cross section (max) occurs at the velocity a E v ~ h For E ~ 0 , max observed v ~ 0 Otherwise, max observed at high v 2) Charge exchange involving molecular species Exoergic charge exchange (E < 0) Release of E as product vibration Energetically nearly resonant large at near thermal velocity Endoergic charge exchange (E > 0) Small at near thermal velocity. Usually keV impact energy needed. Reactant vibrational energy sometimes helps to increase , but not dramatically. For near thermal collision Exoergicity rule large when E 0 small when E > 0 3. Instrumentation 1) Requirement Collision cell for conventional tandem mass spectrometry M G m1 , m , etc., 2 fragment ions from M G M , m1 , m , etc M 2 Charge exchange M G M G For charge exchange at low impact energy, M+ must be decelerated. Should detect G+, which moves thermally inside the cell. Low yield. 2) Instrumentation Second collision cell EM Ion beam Electric sector Conversion Magnetic sector dynode First collision cell Conversion dynode First collision cell Collision Cell Y-lens Ion Source Repeller 3) First Cell Ion source Collision cell M , m1 , G Magnetic M+ analyzer Vs Vc Type I ions ( formed by EI in the source) KI = eVs Type II ions ( formed by CID in the cell) KII = e [Vs+(m1/M)(Vs-Vc)] Type III ions ( formed from collision gas) KIII = eVc Magnetic analyzer : m/z = B2r2e2/2K 4) Second Cell Ion source Magnetic analyzer Collision cell Electrostatic analyzer M1 , M , 2 M1 Vs Vc Select M 1 by magnetic analyzer. Measure ion kinetic energy by electrostatic analyzer. Detect ions generated from collision gas ( KE of type III differs from those of Type I & II) 4. Charge exchange data for C6H6+ 1) Second cell + ~ 77 (MID) RE (C6H6+, A 2 E 2g ) = 11.488 eV IE (CS2) =10.07 eV II E = 10.07-11.488 = -1.418 eV Intensity Exoergic ! II Ion signal from collision gas observed at eVc III II Lifetime 20s or longer. 3900 3930 3960 3990 Translational Energy, eV 2) First cell I I I I ~ RE (C6H6+, A 2 E 2g ) = 11.488 eV IE (CS2) =10.07 eV IE (CH3Cl) = 11.28 eV I Exoergic ! Ion signals from collision gas observed and can be identified. I III I I I 3) Relative yield of collision gas ions vs. impact energy ~ When A2E2g state is fully quenched RE ( C6H6+, X 2 E1g ) = 9.243 eV ~ -1 10 Relative Yield, (A ) / (C6H6 ) IE, eV . + + 1,3-C4H6 + -2 CS2 10 + CH3Cl + 1,3-C4H6 9.08 CH3F . -3 + + 10 CH4 CS2 10.07 -4 10 CH3Cl 11.28 -5 10 CH3F 12.47 -6 10 CH4 12.51 0 200 400 600 800 1000 Primary Ion Translational Energy, eV ~ When A2E2g state is present ~ RE ( C6H6+, A2 E 2g ) = 11.488 eV -1 10 IE, eV Relative Yield, (A ) / (C6H6 ) +¡¤ . + 1,3-C4H6 + -2 CS2 10 CH3Cl + 1,3-C4H6 9.08 + CH3F -3 . + 10 + CH4 CS2 10.07 -4 10 CH3Cl 11.28 -5 10 CH3F 12.47 10 -6 CH4 12.51 0 200 400 600 800 Primary Ion Translational Energy, eV 4. Summary Collision gas ion yield is dramatically enhanced when the charge exchange is exoergic. Detect charge exchange signal for various collision gases with different IE Presence / absence of a very long –lived state. Estimation of its RE. Or, charge exchange energy titration technique to probe excited electronic states. IV. Benzene derivatives J. Chem. Phys. In press, 2002. A. Halobenzenes C6H6 C6H5X X 6b2 (Xnp∥ character) 2b1 (Xnp⊥ character) 3b1 - 1e1g - + 1a2 + 6b2 2b1 np ~ e- removal from 3b1 (3b1)-1X 2 B 1 1a2 (1a2) A A -1 ~ 2 Hole states appearing in ~ 2 6b2 (6b2)-1 B 2 B photoelectron spectra 2 -1 ~ 2 2b1 (2b1) C B 1 C6H5Cl Photoelectron Spectrum ~ B ~ X ~ 2 2 ~ Widths of vibrational bands of B B 2 & X B1 are comparable. ~ Possibility of very long lifetime for B 2 B 2 of C6H5Cl+ C6H5Br Photoelectron Spectrum ~ B ~ X ~2 2 ~ Widths of vibrational bands of B B 2 & X B1 are comparable. ~ Possibility of very long lifetime for B 2 B 2of C6H5Br+ C6H5I Photoelectron Spectrum ~ ~ B 2 B 2 bands broader than X 2 B1 ~ Rapid relaxation of B 2 B 2 of C6H5I+ C6H5F Photoelectron Spectrum (F2p∥)-1 ~ X ~2 (F2p∥)-1 bands broader than X B1 Rapid relaxation B. Triple bonds C6H6 C6H5CN/ C6H5CCH CX 3b1 1e1g 1a2 6b2 2b1 6b2 (CX∥) character 2b1 (CX⊥) character e- removal from 3b1 ~ X 2 B1 1a2 ~2 Hole states appearing in A A2 6b2 ~2 photoelectron spectra B B2 ~ B ~ X C6H5CCH Photoelectron Spectrum ~ ~ X B C6H5CN Photoelectron Spectrum ~ Sharp vibrational bands for B 2 B 2 states. ~ Possibility of very long-lived B B 2 states of 2 C6H5CN+, C6H5CCH+. C. Experimental results 1) C6H5Cl+ ~ C6H5Cl+( B) + CH3Cl C6H5Cl + CH3Cl+ ~ RE (C6H5Cl+, B ) = 11.330 eV IE (CH3Cl) =11.28 eV E = 11.28 eV – 11.330 eV = -0.05 eV, exoergic! ~ CH3Cl+ would be observed if B of C6H5Cl+ is very long-lived. (a) + I C4H3 100 50 . + . I C4H2 + I C4H4 0 (b) + Partial mass spectrum of C6H5Cl generated by 20 eV EI I C4H3 recorded under the single focusing condition with 4006 eV Relative Intensity 100 acceleration energy is shown in (a). (b) and (c) are mass . spectra in the same range recorded with CH3Cl in the + . . collision cell floated at 3910 and 3960 V, respectively. III CH3 Cl 50 + + + . 35 I C4H2 + III CH2 Cl III CH337Cl III CH2 Cl 35 + Type II signals at m/z 49.3 and 50.3 in (b) and at m/z 49.6 37 I C4H4 II and 50.6 in (c) are due to collision-induced dissociation of II II 0 C6H5Cl+ to C4H2+ and C4H3+, respectively. The peaks at (c) + m/z 50.6 in (b) and at m/z 50.8 in (c) are due to collision- I C4H3 100 induced dissociation of C6H5+ to C4H3+. . + + III CH335Cl II/III CH2 Cl . 37 + 50 . . III CH3 Cl + 37 III CH2 Cl + + C4H2 35 I C4H4 I II II 0 48 49 50 51 52 m/z 2) C6H5Br+ ~ C6H5Br+(B ) + CH3Br C6H5Br + CH3Br+ ~ RE (C6H5Br+, B ) = 10.633 eV IE (CH3Br) =10.54 eV E = 10.54 eV - 10.633 eV = -0.093 eV, exoergic! ~ CH3Br+ would be observed if B of C6H5Br+ is very long-lived. . + . + III CH3 Br III CH381Br 79 Relative Intensity 100 + + III CH279Br III CH2 Br 81 50 0 88 90 92 94 96 m/z Partial mass spectrum obtained under the single focusing condition with C6H5Br and CH3Br introduced into the ion source and collision cell, respectively. C6H5Br was ionized by 20 eV EI and acceleration energy was 4008 eV. Collision cell was floated at 3907 V. 3) C6H5CN+ ~ C6H5CN+(B ) + CH3Cl C6H5CN + CH3Cl+ ~ RE (C6H5CN+, B ) = 11.84 eV IE (CH3Cl) =11.28 eV E = 11.28 eV – 11.84 eV = -0.56 eV, exoergic! ~ CH3Cl + would be observed if B of C6H5CN+ is very long-lived. . + III CH335Cl . . + Relative Intensity I C4H4 + 100 I C4H2 + . III CH235Cl + III CH3 Cl 37 + 50 III CH237Cl II II II II II 0 47 48 49 50 51 52 m/z Partial mass spectrum obtained under the single focusing condition with C6H5CN and CH3Cl introduced into the ion source and collision cell, respectively. C6H5CN was ionized by 20 eV EI and acceleration energy was 4007 eV. Collision cell was floated at 3910 V. Type II signals at m/z 49.3, 50.3, and 51.3 are due to collision-induced dissociation of C6H5CN+ to C4H2+, C4H3+, and C4H4+, respectively. Those at m/z 49.6 and 50.6 are due to collision-induced dissociation of C6H4+ to C4H2+ and C4H3+, respectively. 4) C6H5CCH+ ~ C6H5CCH+(B ) + CS2 C6H5CCH + CS2+ ~ RE (C6H5CCH+, B ) = 10.36 eV IE (CS2) =10.07 eV E = 10.07 eV - 10.36 eV = -0.29 eV, exoergic! ~ CS2+ would be observed if B of C6H5CCH+ is very long-lived. . + III C32S2 Relative Intensity 100 50 . . + III C S S + 32 34 I C6H4 II II 0 70 72 74 76 78 m/z Partial mass spectrum obtained under the single focusing condition with C6H5CCH and CS2 introduced into the ion source and collision cell, respectively. C6H5CCH was ionized by 14 eV EI and acceleration energy was 4006 eV. Collision cell was floated at 3942 V. Type II signals at m/z 73.5 and 75.7 are due to collision-induced dissociation of C6H5CCH+ to C6H2+ and C6H4+, respectively. Collision gases, their ionization energies(IE) in eV , and success / failure to generate their ions by charge exchange with some precursor ions Precursor ion Collision gas IE, eV C6H5Cl+• C6H5Br+• C6H5CN+• C6H5CCH+• C6H5I+• C6H5F+• (CH3)2CHNH2 8.72 O O O O 1,3-C4H6 9.07 O X O (butadiene) CS2 10.07 O CH3Br 10.54 O O O X X C2H5Cl 10.98 X CH3Cl 11.28 O X O X C2H6 11.52 X O O2 12.07 X Xe 12.12 X X X CHF3 13.86 X ~ Recombination energy (X) 9.066 8.991 9.71 8.75 8.754 9.20 ~ Recombination energy (B) 11.330 10.633 11.84 10.36 9.771 13.81* ~ ~ ~ Recombination energies of the X2B1, A2A2, and B2B2 states and the ~ oscillator strengths of the radiative transitions from the B2B2 states. State C6H5Cl+ C6H5Br+ C6H5I+ C6H5CN+ C6H5CCH+ ~2 9.066 8.991 8.754 9.71 8.75 X B1 (0.0000000) (0.0000000) (0.0000000) (0.0000000) (0.0000000) ~2 9.707 9.663 9.505 10.17 9.34 A A2 (0.0000008) (0.0000001) (0.0000000) (0.0000010) (0.0000004) ~2 B B2 11.330 10.633 9.771 11.84 10.36 Lowest quartet 13.236 13.381 12.664 13.3 12.7 Reaction threshold 12.356 11.891 11.07 12.725 12.41 ~ Radiative decay of B2B2 is not efficient for all the cases. ~ B states are not dissociative. ~ The lowest quartet states lie ~2 eV above the B state. Relaxation by doublet – quartet intersystem crossing would not occur. Internal conversion must be inefficient for the ~ states except for B ~ state of C H I+, internal conversion must be efficient. C H I+. For the B 6 5 6 5 V. Vinyl derivatives A. Detection of Type III ions by double focusing mass spectrometry Type I : KI = eVS Type II : KII = e[VC + (m2/m1)(VS - VC)] Type III : KIII = eVC Magnetic analyzer Electrostatic analyzer Ion source Collision cell Vs Vc Scheme 1. Set the electrostatic analyzer (kinetic energy analyzer) to transmit ions with kinetic energy eVc. 2. Scan the magnetic analyzer (momentum analyzer, or mass analyzer). Detect Type III ions only. B. Vinyl halide C2H4 C2H3X X a ( Xnp∥ character) a ( Xnp⊥ character) a a Xnp a ~ e- removal from a (C=C) X 2 A ~ a (Xnp∥) A 2 A Hole states appearing in photoelectron spectra ~ a (Xnp⊥) B 2 B 1) Vinyl chloride ~2 Sharp vibrational bands for A A Possibility of very long lifetime. 2) Vinyl bromide ~ Sharp vibrational bands for A 2 A Possibility of very long lifetime. 3) Vinyl iodide ~ A ~ X ~ Sharp vibrational bands for A 2 A Possibility of very long lifetime. C. CH2=CHCN, Acrylonitrile ~ X ~ A ~ Possibility of very long lifetime for A 2 A D. CH2=CHF, Vinyl fluoride ~ X ~ A ~ Broad A 2 A bands ~ Short lifetime for A 2 A E. Experimental results 1) CH2=CHCl+ ~ CH2=CHCl+(A) + CH3Cl CH2=CHCl + CH3Cl+ ~ RE (CH2=CHCl+, A ) = 11.664 eV IE (CH3Cl) =11.28 eV E = 11.28 eV – 11.664 eV = -0.384 eV, exoergic! ~ CH3Cl+ would be observed if A of CH2=CHCl+ is very long-lived. Relative Intensity (a) I I Single – focusing mass spectrum I recorded for C2H3Cl with CH3Cl introduced to the first cell. I III 20 30 40 50 60 70 III Relative Intensity (b) 35 CH3 Cl + III III Double – focusing mass spectrum III 20 30 40 50 60 70 m/z ~ A state of CH2=CHCl+ is very long-lived. 2) CH2=CHBr+ ~ CH2=CHBr+(A) + CH3Br CH2=CHBr + CH3Br+ ~ RE (CH2=CHBr+, A ) = 10.899 eV IE (CH3Br) =10.54 eV E = 10.54 eV – 10.899 eV = -0.359 eV, exoergic! ~ CH3Br+ would be observed if A of CH2=CHBr+ is very long-lived. (a) III III Relative Intenxity III III I I III 90 92 94 96 98 (b) III III Relative Intensity 79 + 81 + CH3 Br CH3 Br III III III 90 92 94 96 98 m/z ~ A state of CH2=CHBr+ is very long-lived. 3) CH2=CHI+ ~ CH2=CHI+(A ) + CH2=C=CH2 CH2=CHI + CH2=C=CH2+ ~ RE (CH2=CHI+, A ) = 10.08 eV IE (allene : CH2=C=CH2) =9.69 eV E = 9.69 eV – 10.08 eV = -0.39 eV, exoergic! ~ CH2=C=CH2 + would be observed if A of CH2=CHI+ is very long-lived. III (a) Relative Intensity III III 35 37 39 41 43 III (b) + C3H4 Relative Intensity III III 35 37 39 41 43 m/z ~ A state of CH2=CHI+ is very long-lived. 4) CH2=CHCN+ ~ CH2=CHCN+(A) + Xe CH2=CHCN + Xe+ ~ RE (CH2=CHCN+, A ) = 12.36 eV IE (Xe) =12.12 eV E = 12.12 eV – 12.36 eV = -0.24 eV, exoergic! ~ Xe+ would be observed if A of CH2=CHCN+ is very long-lived. III III (a) III Relative Intensity III III III III 124 126 128 130 132 134 136 138 140 III III (b) III Relative Intensity III III III III 124 126 128 130 132 134 136 138 140 m/z ~ A state of CH2=CHCN+ is very long-lived. 5) CH2=CHF+ ~ CH2=CHF+( A) + CH3F CH2=CHF + CH3F+ ~ RE (CH2=CHF+, A ) = 13.80 eV IE (CH3F) =12.50 eV E = 12.50 eV – 13.80 eV = -1.3 eV, exoergic! ~ CH3F+ would be observed if A of CH2=CHF+ is very long-lived. (a) I Relative Intensity Mass spectrum of C2H3F generated by 20 eV EI recorded under the single focusing condition without CH3F. 20 30 40 50 (b) I Relative Intensity Mass spectrum of C2H3F generated by 20 eV EI recorded under the single focusing condition with CH3F. I I 20 30 40 50 m/z ~ A state of CH2=CHF+ is not long-lived. Precursor ions Collision gas IE, eV C2H3Cl+• C2H3Br+• C2H3I+• C2H3CN+• C2H3F+• 1,3-C4H6 9.07 O O O O (butadiene) C 3 H4 9.692 O (Allene) CH3Br 10.54 O O X O CH3Cl 11.28 O X X Xe 12.12 X O CH3F 12.50 X X Ar 15.76 X ~ Recombination energy (X) 10.005 9.804 9.35 10.91 10.63 ~ Recombination energy (A) 11.664 10.899 10.08 12.36 13.80 VI. Conclusion 1. Charge exchange ionization has been developed as a useful technique to find very long-lived excited electronic states of polyatomic ions and estimate their recombination energies. 2. The following very long-lived excited electronic states have been found. +, ~2 ~ +, A 2 A C 6H 6 A E 2g CH2CHCl ~2 ~ C6H5Cl +, B B2 CH2CHBr+, A 2 A ~2 ~ C6H5Br +, B B2 CH2CHI+, A 2 A +, ~2 ~ +, A 2 A C6H5CN B B2 CH2CHCN ~2 C 6H 5 CCH+, B B2 Much more than found over the past 50 years!
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