Supplementary Material for

H-atom reactivity as a function of temperature in solid parahydrogen: The H + N2O reaction

Fredrick M. Mutunga, Shelby E. Follett and David T. Anderson

Department of Chemistry, University of Wyoming, Laramie, WY 82071

Email: danderso@uwyo.edu

1. Experimental details

The N2O doped pH2 crystals are prepared using the rapid vapor deposition method of

Fajardo and Tam.1, 2 The crystal is grown by co-deposition of independent gas flows of 15N218O

and pH2 onto a pre-cooled BaF2 optical substrate held at ~2.5 K within a sample-in-vacuum

liquid-He cryostat. We estimate the N2O dopant concentrations based on the measured flow

rates of dopant and pH2 gas into the cryostat during deposition. For these studies the dopant

concentration ranged from 20 to 70 ppm. The pH2 solids are prepared by enriching normal-H2

gas to greater than 99.97% pH2 enrichment levels using a variable temperature ortho/para

converter operated near 14.0 K. The orthohydrogen (oH2) concentration in the sample can be

checked using the integrated intensity of the oH2-induced Q1(0) feature1 and the measured

crystal thickness.3 The temperature of the sample is measured using two Si-diode sensors; one

is mounted (TA) to the cold tip of the helium cryostat and the other (TB) is mounted to the Au-

plated oxygen-free-high-conductivity Cu substrate holder at the point furthest from the cold

tip. All the reported temperatures are using TB.

The H-atoms are generated as by-products in the 193 nm in situ photolysis of the 15N218O

precursor. The unfocused 193 nm output of a broadband ArF excimer laser (Gam Laser EX5)

with 8 ns pulse duration is directed at an angle of 45° with respect to the surface normal of the

BaF2 optical substrate. The FTIR beam is focused with 8″ off-axis parabolic mirrors through the

sample at normal incidence to the BaF2 optical substrate in a transmission optical setup. This

optical setup permits FTIR spectra to be recorded within the photolysis region either during or

immediately after 193 nm irradiation. The laser fluences in these experiments, measured with

a power meter after an adjustable iris located in front of the photolysis window on the cryostat,

range from 60 to 75 J cm-2 pulse-1 and we used the highest 250 Hz repetition rate of the laser.

High-resolution rapid scan FTIR spectroscopy (e.g., acquisitions times of 290 sec for 16

co-added scans at 0.03 cm-1 resolution) is performed on the sample using a FTIR spectrometer

(Bruker IFS 120HR) equipped with a glowbar source and Ge-coated KBr beamsplitter. For all the

kinetic measurements we use a liquid nitrogen cooled HgCdTe detector to record spectra from

700 to 4500 cm-1. The concentration of the cis and trans isomers of H15N15N18O are measured

from the integrated intensity of the 2 (NN stretch) absorptions near 1575 cm-1 using the

following integration protocols. The cis peak is integrated from 1573.5 to 1576.5 cm-1 and the

trans peak from 1577.5 to 1580.5 cm-1. We calculate the concentration in units of parts per

million (ppm) using the following equation,

[ X ]=2.303∫ log10( I /I 0 )d~ν

ε lV 0(1 x10

6 )(1)

where X is cis or trans, is the integrated absorption coefficient, l is the IR pathlength through

the crystal, and V0 is the molar volume4 of solid pH2 at liquid helium temperatures (23.16 cm3

mol-1). The value of l is determined for each sample using the IR spectroscopic method

developed by Fajardo.3 The values used are 367.3 and 240.2 km mol-1 for the cis and trans 2

modes, respectively. Note these values are for the H15N15N18O isotopomer and were provided

to us by Professor Kirk A. Peterson.5

2. Spectroscopic results

We made the cis- and trans-H15N15N18O spectroscopic assignments based on the

agreement with the peak positions of Xe matrix isolation results6, 7 and full anharmonic

calculations of the vibrational frequencies and isotopic shifts.8 The measured peak positions

and isotopic shifts are presented in Tables S1 and S2. We tested these assignments using binary

intensity correlation plots which are shown in Figures S1 and S1. These correlation plots

demonstrate that there are two distinct product species and that the peak assignments to a

given isomer are internally consistent. Shown in Figure S3 is a series of IR spectra recorded

during the course of a single photolysis experiment on a 15N218O doped pH2 solid conducted at

1.81(2) K showing the corresponding cis- and trans-H15N15N18O peaks used to monitor the

chemistry. We chose to use the 2 mode (NN stretch) to monitor the concentration of cis- and

trans-H15N15N18O. Trace (a) in Figure S3 is recorded prior to photolysis and shows no discernible

IR absorptions in this small window between 1570 and 1585 cm -1. Trace (b) is recorded

beginning immediately after the 10 minute photolysis of the sample at 1.84 K and shows a small

peak at 1574.91 cm-1 due to the slight production of cis during low temperature photolysis.

Trace (c) is recorded 486.4 minutes after trace (b) while maintaining the sample at a constant

temperature of 1.81(2) K. Clearly there is significant growth in the intensity of the two peaks

corresponding to cis- and trans-H15N15N18O. We use the integrated intensity of these two peaks

combined with the measured thickness of the sample to calculate the concentration of both

isomers. Note that the FTIR spectra shown in Figure S3 are for the experiment displayed in

Figure 1.

Table S1. Measured peak positions and isotopic shifts () all in cm-1 for cis-HNNO.

mode 14N216O/Xea 14N2

16O/pH215N2

18O/pH2 (Expt.) (Calc.)b

1 NH str 3158 3069.08 -6.72 NN str 1623.7 1629.28 1574.91 -54.37 -55.43 NO str 1273.4 1282.48 1255.31 -27.17 -26.5

4 HNN bend 1166.9 -40.0aIn a Xe matrix, Ref. 7. bCalculated isotopic shift, Ref. 8.

Table S2. Measured peak positions and isotopic shifts () all in cm-1 for trans-HNNO.

mode 14N216O/Xea 14N2

16O/pH215N2

18O/pH2 (Expt.) (Calc.)b

1 NH str 3254.0 3280.29 3273.17 -7.12 -7.12 NN str 1628.9 1634.52 1578.82 -55.70 -57.83 NO str 1294.5 1299.19 1272.26 -26.93 -35.3

4 HNN bend 1213.4 1221.27 1186.91 -34.36 -31.1aIn a Xe matrix, Ref. 7. bCalculated isotopic shift, Ref. 8.

Figure S1. Binary intensity correlation plot for cis-H15N15N18O.

Figure S2. Binary intensity correlation plot for trans-H15N15N18O.

Figure S3. Representative IR spectra displaying the 2 (NN stretch) peaks of cis- and trans-H15N15N18O at various steps in a low temperature reaction study on a 0.24(2) cm thick, 23 ppm 15N2

18O doped pH2 sample. Trace (a) is recorded at 1.82 K before photolysis, trace (b) is recorded (16 co-added scans, 290 sec, 0.03 cm-1 resolution) at 1.83 K right after photolysis, and trace (c) is recorded 486.4 min after trace (b) while the sample is maintained at a constant temperature of 1.81(2) K.

wavenumber / cm-1

1570 1575 1580 1585

log 1

0(I 0

/I)

/ ab

s

0.0

0.1

0.2

0.3

0.4

0.5

cis

trans

(a)

(b)

(c)

Table S3. Kinetic parameters determined from fit to data in Figure 1.

parameter cisa transb

A10 / ppm 6.01(24)(28) 2.60(22)A20 / ppm 0.0440(28) 0.000(8)

k1 /10-3 min-1 1.413(46) 1.67(22)k2 /10-3 min-1 8.63(18) 12.1(27)

R2 0.999162 0.999773

acis function:f ( t )=A20 exp(−k 2 t )+

k1 A10

(k1−k2)(exp (−k2 t )−exp(−k1 t ))

btrans function:

f ( t )=A10(1−exp(−k1 t )− k1

(k2−k1 ) (exp(−k1 t )−exp(−k2 t )))+A20 (1−exp(−k2 t )).

REFERENCES

[1] S. Tam, and M. E. Fajardo, Rev. Sci. Instrum. 70, 1926 (1999).

[2] S. Tam, and M. E. Fajardo, Appl. Spectrosc. 55, 1634 (2001).

[3] M. E. Fajardo, in Physics and Chemistry at Low Temperatures, edited by L. Khriachtchev (Pan

Stanford Publishing Pte. Ltd., Singapore, 2011), pp. 167.

[4] I. F. Silvera, Rev. Mod. Phys. 52, 393 (1980).

[5] K. A. Peterson, (private communication, 2012).

[6] K. M.-R. Isokoski, in Ph.D. Thesis (University of Helsinki, Helsinki, 2008), p. 69.

[7] S. L. Laursen, A. E. Delia, and K. Mitchell, J. Phys. Chem. A 104, 3681 (2000).

[8] K. A. Peterson, and J. S. Francisco, J. Chem. Phys. 134, 084308 (2011).