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Changing the wavelength of the probing pulse can allow not just the final product, free CN, to be determined but the intermediates along the reaction path including the transition state.
For NaI one can see the activated complex vibrate at (27 cm-1) 1.25 ps intervals surviving for 10 oscillations– see fig. 27.9 Atkins 6th ed.
19Fast flow tubes; 1 m3/s, inert coating, t=d/v In a RF discharge: O2 O + Oorpass H2 over heated
tungsten filament or O3 over 1000C quartz, etc.
Use non-invasive methods for analysis eg absorption, emissionGas titration: add stable NO2 (measurable flow rate) Fast O+NO2 NO+O2 then O+NO NO2
NO2h
End-point? Lights out when flow(NO2) = flow(O)
N O 2
O 2
20ClO + NO3 J. Phys. Chem. 95:7747 (1991) 1.5 m long, 4 cm od, Pyrex tube with sliding injector to vary
reaction time F + HNO3 NO3 + HF [NO3] monitor at 662 nm F + HCl Cl + HF followed by Cl + O3 ClO + O2
M S
He
F 2 / He
HC l
He
HNO 3 / HeF 2 / He
SLIDING INJE CTOR
RF
21Problem [RK, Pilling & Seakins, p36]
HO2 + C2H4 C2H5
+ O2 C2H5O2
MS determines LH channel 11%, RH channel 89%C2H5 signal 6.14 3.95 2.53 1.25 0.70 0.40
Injector d / cm 3 5 7 10 12 15Linear flow velocity was 1,080 cm s-1 at 295 K & 263 Pa.Calculate 1st order rate constant; NB [O2]0>>[C2H5
]0
Either convert d’s into times via flow rate and then plot ln(signal) versus t
Or plot ln(signal) vs d & convert slope
22Flow tubes; pros & cons Mixing time restricts timescale to millisecond range Difficult to work at pressures > (atm/100) Wall reactions can complicate kinetics
– coat with Teflon or halocarbon wax; or vary tube diameter Cheap to build & operate, sensitive detection available
– Resonance fluorescence– Laser induced fluorescence– Mass spectrometry– Laser magnetic resonance
23Resonance fluorescence Atomic species (H, N, O, Br, Cl, F) mainly not molecular Atomic lines are very narrow; chance of absorption by
another species is highly unlikely Resonance lamp: mcwe discharge dissociates H2
H atoms formed in electronically excited state; fluoresce, emitting photon which H-atoms in rxn vessel absorb & re-emit them where they can be detected by PMT
Lamp: H2 H H* H + h
Rxn cell: H + h H* H + h
24LIF; detection of OH Excitation pulse at 282 nm to
upper state of OH with lifetime of ns; fluorescence to ground state at 308 nm
IF n relative concentrations not
absolute (drawback). Right angle geometry Good candidates:
– CN, CH, CH3O, NH, H, SO
v '= 2
v '= 1
v '= 0
v ''= 2
v ''= 1
v ''= 0
2 8 2 n m 3 0 8 n m
25Reactions in shock waves
Wide range of T’s & P’s accessible; 2,000 K, 50 bar routine Thermodynamics of high-T species eg Ar up to 5,000 K Study birth of compounds: C6H5CHO CO* + C6H6
Energy transfer rxns.: CO2 + M CO2* + M Relative rates, use standard rxn as “clock”
o sc illo sco p eh e liu m
h e liu m
va cu u m
va cu u m va cu u m
d ia p h ra g m s d r ive n se c tio nd r ive r se c tio n
co m p u te r
m o n o ch ro m a to r
co m p o u n d
26Mode of action of shock tube Fast bunsen-burner (ns)
Shock wave acts as a piston compressing & heating the gas ahead of it
Study rxns behind incident shock wave or reflected shock wave (milli-s times)
Non-invasive techniques T & p by computation from
measured shock velocity
P
D IS T A N C E
T 1
T 2
T 3
T
27Problem A single-pulse shock tube used to study 1st order rxn
C2H5I C2H4 + HI; to avoid errors in T measurement a comparative study of a standard cpd. S was carried out with C3H7I C3H6 + HI for which kS=9.11012 exp(-21,900/T) s-1. For a rxn time of 220 s 5% decomp. of C3H7I or S occurred.
What was the temp. of the shock wave? [900 K] For C2H5I 0.90% decomp. occurred; evaluate k If at 800 K (k/kS) = 0.102 compute the Arrhenius