Anomalous CH Stretch Intensity Effects in Halomethyl Radicals: “Charge-Sloshing” vs. Bond- Dipole Contributions to IR Transition Moments E.S. Whitney, F. Dong, C. Savage, D.J. Nesbitt Molecular Spectroscopy Symposium The Ohio State University Columbus, OH June 20, 2006 JILA, Department of Chemistry and Biochemistry National Institute for Standards and Technology University of Colorado Boulder, CO E.S. Whitney, F. Dong, C. Savage, D.J. Nesbitt Molecular Spectroscopy Symposium The Ohio State University Columbus, OH June 20, 2006 JILA, Department of Chemistry and Biochemistry National Institute for Standards and Technology University of Colorado Boulder, CO JILA

ExperimentalExperimental Sub-Doppler molecular linewidths (  40 MHz in Ne expansion) Servoloop locked optical transfer cavities for high frequency precision (  20 MHz) Shot noise limited detection sensitivity: 1.5 x (N min  10 7 #/cm 3 /qs) Sub-Doppler molecular linewidths (  40 MHz in Ne expansion) Servoloop locked optical transfer cavities for high frequency precision (  20 MHz) Shot noise limited detection sensitivity: 1.5 x (N min  10 7 #/cm 3 /qs)

Slit Discharge in Action High radical densities at slit orifice (  /cm 3 !)High radical densities at slit orifice (  /cm 3 !) “Maximally simplified” conditions for high resolution spectroscopy of cold but highly reactive species“Maximally simplified” conditions for high resolution spectroscopy of cold but highly reactive species Intense (  A), stable and confined discharges at  500 TorrIntense (  A), stable and confined discharges at  500 Torr Highly localized (1 mm) discharge (  10 5 cm/s,  1  s transit time)Highly localized (1 mm) discharge (  10 5 cm/s,  1  s transit time) Insufficient time for further radical chemistry!Insufficient time for further radical chemistry! High radical densities at slit orifice (  /cm 3 !)High radical densities at slit orifice (  /cm 3 !) “Maximally simplified” conditions for high resolution spectroscopy of cold but highly reactive species“Maximally simplified” conditions for high resolution spectroscopy of cold but highly reactive species Intense (  A), stable and confined discharges at  500 TorrIntense (  A), stable and confined discharges at  500 Torr Highly localized (1 mm) discharge (  10 5 cm/s,  1  s transit time)Highly localized (1 mm) discharge (  10 5 cm/s,  1  s transit time) Insufficient time for further radical chemistry!Insufficient time for further radical chemistry!

Concentration Modulation localized discharge

1:3 ( 37 Cl: 35 Cl) Multiple Frequency Scales High resolution rovibration, spin-rotation and (partial) hyperfine information on ground state and vibrationally excited radicalsHigh resolution rovibration, spin-rotation and (partial) hyperfine information on ground state and vibrationally excited radicals P K=1 /P K=0  1:3 due to para/ortho H nuclear spin statistics P K=1 /P K=0  1:3 due to para/ortho H nuclear spin statistics Reversal from P J=0 /P J=1  3:1 in H 2 … Reversal from P J=0 /P J=1  3:1 in H 2 … …consistent with Pauli Principle for A´´ electronic state …consistent with Pauli Principle for A´´ electronic state

Fluoromethyl Radical Previous matrix isolation, REMPI, FIR-LMR, IR diode, and microwave studiesPrevious matrix isolation, REMPI, FIR-LMR, IR diode, and microwave studies First high resolution gas-phase characterization in CH 2 stretching regionFirst high resolution gas-phase characterization in CH 2 stretching region Previous matrix isolation, REMPI, FIR-LMR, IR diode, and microwave studiesPrevious matrix isolation, REMPI, FIR-LMR, IR diode, and microwave studies First high resolution gas-phase characterization in CH 2 stretching regionFirst high resolution gas-phase characterization in CH 2 stretching region Symmetric CH 2 stretch Antisymmetric CH 2 stretch

Fluoromethyl Symmetric CH 2 Stretch A-type bandA-type band Clear P,R branch progressions at 18KClear P,R branch progressions at 18K Readily assignable via 2-line combination differences (Hirota et al.)Readily assignable via 2-line combination differences (Hirota et al.) Peak absorbances on order of %Peak absorbances on order of % A-type bandA-type band Clear P,R branch progressions at 18KClear P,R branch progressions at 18K Readily assignable via 2-line combination differences (Hirota et al.)Readily assignable via 2-line combination differences (Hirota et al.) Peak absorbances on order of %Peak absorbances on order of % Symmetric CH 2 stretch

Fluoromethyl Antisymmetric Stretch B-type bandB-type band P, Q, R branch structure at 18KP, Q, R branch structure at 18K Peak absorbances on order of %Peak absorbances on order of % 2-fold weaker band intensity than symmetric stretch)2-fold weaker band intensity than symmetric stretch) B-type bandB-type band P, Q, R branch structure at 18KP, Q, R branch structure at 18K Peak absorbances on order of %Peak absorbances on order of % 2-fold weaker band intensity than symmetric stretch)2-fold weaker band intensity than symmetric stretch) Antisymmetric CH 2 stretch

Bond-Dipole Expectations Bond dipole model predicts:Bond dipole model predicts: A-type symmetric and B-type antisymmetric stretch bands (in agreement with expt…)A-type symmetric and B-type antisymmetric stretch bands (in agreement with expt…) …but I sym /I antisym  cot 2 (60 o ) = 1/3…but I sym /I antisym  cot 2 (60 o ) = 1/3 cf. (I sym /I antisym ) expt = 1.8(2):1cf. (I sym /I antisym ) expt = 1.8(2):1  6-fold enhancement of symmetric to antisymmetric stretch intensity ratio  6-fold enhancement of symmetric to antisymmetric stretch intensity ratio Bond dipole model predicts:Bond dipole model predicts: A-type symmetric and B-type antisymmetric stretch bands (in agreement with expt…)A-type symmetric and B-type antisymmetric stretch bands (in agreement with expt…) …but I sym /I antisym  cot 2 (60 o ) = 1/3…but I sym /I antisym  cot 2 (60 o ) = 1/3 cf. (I sym /I antisym ) expt = 1.8(2):1cf. (I sym /I antisym ) expt = 1.8(2):1  6-fold enhancement of symmetric to antisymmetric stretch intensity ratio  6-fold enhancement of symmetric to antisymmetric stretch intensity ratio Symmetric CH 2 stretch Antisymmetric CH 2 stretch ++ ++ --  ++ ++ -- 

“Charge-Sloshing”“Charge-Sloshing” Electron withdrawing due to halogen substitution results in strong decrease in H atom  + charge with CH bond extensionElectron withdrawing due to halogen substitution results in strong decrease in H atom  + charge with CH bond extension “Charge sloshing” contributions in opposition to bond-dipole expectations“Charge sloshing” contributions in opposition to bond-dipole expectations Dipole moment derivative reverses directions from “normal” bond-dipole pictureDipole moment derivative reverses directions from “normal” bond-dipole picture Electron withdrawing due to halogen substitution results in strong decrease in H atom  + charge with CH bond extensionElectron withdrawing due to halogen substitution results in strong decrease in H atom  + charge with CH bond extension “Charge sloshing” contributions in opposition to bond-dipole expectations“Charge sloshing” contributions in opposition to bond-dipole expectations Dipole moment derivative reverses directions from “normal” bond-dipole pictureDipole moment derivative reverses directions from “normal” bond-dipole picture

Bend Angle Dependence Sym/antisym intensity ratios from ab initio calculations (B3LYP/aug- cc-PVTZ)Sym/antisym intensity ratios from ab initio calculations (B3LYP/aug- cc-PVTZ) Strong dependence of intensities on bend angleStrong dependence of intensities on bend angle I sym /I antisym = 1.8(2):1 consistent with  29(4) oI sym /I antisym = 1.8(2):1 consistent with  29(4) o Intensity based expt’l support for non-planar equilibrium geometryIntensity based expt’l support for non-planar equilibrium geometry Sym/antisym intensity ratios from ab initio calculations (B3LYP/aug- cc-PVTZ)Sym/antisym intensity ratios from ab initio calculations (B3LYP/aug- cc-PVTZ) Strong dependence of intensities on bend angleStrong dependence of intensities on bend angle I sym /I antisym = 1.8(2):1 consistent with  29(4) oI sym /I antisym = 1.8(2):1 consistent with  29(4) o Intensity based expt’l support for non-planar equilibrium geometryIntensity based expt’l support for non-planar equilibrium geometry

Chloromethyl Radical Symmetric CH 2 stretch readily observed in CH 2 Cl (S/N > 50:1)Symmetric CH 2 stretch readily observed in CH 2 Cl (S/N > 50:1) Antisymmetric CH 2 stretch in CH 2 Cl not observed at current sensitivities! (intensity weaker by at least 20-fold)Antisymmetric CH 2 stretch in CH 2 Cl not observed at current sensitivities! (intensity weaker by at least 20-fold) Symmetric CH 2 stretch readily observed in CH 2 Cl (S/N > 50:1)Symmetric CH 2 stretch readily observed in CH 2 Cl (S/N > 50:1) Antisymmetric CH 2 stretch in CH 2 Cl not observed at current sensitivities! (intensity weaker by at least 20-fold)Antisymmetric CH 2 stretch in CH 2 Cl not observed at current sensitivities! (intensity weaker by at least 20-fold) symmetric stretch

“Charge-Sloshing” Again! Less electron withdrawing Cl vs F atom substituentLess electron withdrawing Cl vs F atom substituent Nearly perfect cancellation (down by > 30-fold) of bond-dipole and “charge-sloshing” contributions for CH 2 Cl vs CH 2 F asymmetric CH 2 stretchNearly perfect cancellation (down by > 30-fold) of bond-dipole and “charge-sloshing” contributions for CH 2 Cl vs CH 2 F asymmetric CH 2 stretch Less electron withdrawing Cl vs F atom substituentLess electron withdrawing Cl vs F atom substituent Nearly perfect cancellation (down by > 30-fold) of bond-dipole and “charge-sloshing” contributions for CH 2 Cl vs CH 2 F asymmetric CH 2 stretchNearly perfect cancellation (down by > 30-fold) of bond-dipole and “charge-sloshing” contributions for CH 2 Cl vs CH 2 F asymmetric CH 2 stretch Bond-dipole “Charge-sloshing” CH 2 Cl CH 2 F

ElectronegativityElectronegativity Ab initio intensities (B3LYP/aug-cc-PVTZ)Ab initio intensities (B3LYP/aug-cc-PVTZ) Clear trends in asymmetric/symmetric CH 2 stretch intensity with increasing electronegativity of substituentClear trends in asymmetric/symmetric CH 2 stretch intensity with increasing electronegativity of substituent Near perfect cancellation in antisymmetric CH 2 stretch for CH 2 ClNear perfect cancellation in antisymmetric CH 2 stretch for CH 2 Cl Ab initio intensities (B3LYP/aug-cc-PVTZ)Ab initio intensities (B3LYP/aug-cc-PVTZ) Clear trends in asymmetric/symmetric CH 2 stretch intensity with increasing electronegativity of substituentClear trends in asymmetric/symmetric CH 2 stretch intensity with increasing electronegativity of substituent Near perfect cancellation in antisymmetric CH 2 stretch for CH 2 ClNear perfect cancellation in antisymmetric CH 2 stretch for CH 2 Cl

SummarySummary Strong deviations in bond-dipole IR intensity patterns for halomethyl radicalsStrong deviations in bond-dipole IR intensity patterns for halomethyl radicals “Charge-sloshing” effects from electron withdrawing substituents“Charge-sloshing” effects from electron withdrawing substituents Competition and even sign reversal of IR transition moment for CH 2 stretch vibrationsCompetition and even sign reversal of IR transition moment for CH 2 stretch vibrations Near perfect cancellation of bond-dipole and charge-sloshing intensity contributions predicted and confirmed experimentally for CH 2 Cl asymmetric CH 2 stretchNear perfect cancellation of bond-dipole and charge-sloshing intensity contributions predicted and confirmed experimentally for CH 2 Cl asymmetric CH 2 stretch Strong deviations in bond-dipole IR intensity patterns for halomethyl radicalsStrong deviations in bond-dipole IR intensity patterns for halomethyl radicals “Charge-sloshing” effects from electron withdrawing substituents“Charge-sloshing” effects from electron withdrawing substituents Competition and even sign reversal of IR transition moment for CH 2 stretch vibrationsCompetition and even sign reversal of IR transition moment for CH 2 stretch vibrations Near perfect cancellation of bond-dipole and charge-sloshing intensity contributions predicted and confirmed experimentally for CH 2 Cl asymmetric CH 2 stretchNear perfect cancellation of bond-dipole and charge-sloshing intensity contributions predicted and confirmed experimentally for CH 2 Cl asymmetric CH 2 stretch

AcknowledgementAcknowledgement Feng Dong Erin Whitney David Nesbitt Feng Dong Erin Whitney David Nesbitt Anne McCoy NSFNISTNSFNIST