What is the role of hydrogen bonds in organic acentric material to increase their NLO (SHG) property?
I am wondering if you mean nonlinear optics ( NLO) and second harmonic generation (SHG)? A paper I found on thiourea doped L-threonine single crystal vs the urea doped crystal might lend some clues. It dates to 2010 conference(RSTSCC), Umamaheshwari,R., etal.
The role of H-bonding disruption by urea as a chaotrope is well established, and thiourea is just enough different that the effect might be relegated to this. I am not familiar enough with the quantum chemical description for NLO SHG to guess if this is treated by eigenfunctions, but might guess that someone does know.
It is to be noted that the so called thiourea doped L-threonine single crystal reported by Umamaheshwari,R. first in a conference and later in Paripex Indian J of Physics http://theglobaljournals.com/paripex/file.php?val=November_2013_1384851215_52d50_85.pdf \ is actually L-threonine which can be evidenced from the unit cell data. Many such claims of growth of thiourea doped crystals are erroneous as has been shown for L-alanine thiourea and L-valine thiourea recently. http://link.springer.com/article/10.1007%2Fs10973-015-4501-8
In view of their dubious nature, thiourea doped amino acid crystals will not provide any useful clue for the role of hydrogen bonds in organic acentric material to increase their NLO (SHG) property.
In the literature several authors have claimed that the NLO property is due to H-bonding, although no valid reason or scientifc explanation has been provided as to how H-bonding actually is responsible the SHG output.
In most cases authors have overlooked certain very simple aspects of weak interactions in organic solids. The organic compound 1-formyl-3-thiosemicarbazide exhibits five varieties of hydrogen bond namely N-H...O, N-H...S, N-H...N, C-H...O, and C-H...S (pl. see http://scripts.iucr.org/cgi-bin/paper?lc5030 ) and crystallizes in the monoclinic P21/c space group.
If H-bonds can increase SHG output then one should expect a large SHG signal for 1-formyl-3-thiosemicarbazide. But this is not the case despite the five varieties of H-bond, because It is well known that a compound crystallizing in centrosymmetric space group (P21/c) cannot give any SHG output. The point to be noted is that in organic crystals which contain H-donors due to the presence of -OH or -NH2 and H-acceptors like amino N, carbonyl oxygen, F, Cl, Br etc. are expected to exhibit H-bonding which is always seen in most of the solids. In the absence of -H atoms attached to O or N in a crystal structure one can expect weak C-H...O interactions which is well studied and well documented for many solids. The naturally occuring amino acids (excepting glycine) form several new compounds (some of these are hydrates) which still contain some H atoms attached to the amino N or in some compounds prepared using strong acids the -COO group is protonated. All these compounds are acentric because of the chiral nature of amino acids and exhibit several H-bonds which have been reported in many structurally charcterized both the (zwitter ionic) free amino acids and several of their compounds. The H-bonding in these compounds is a consequence of the presence of H-donors and H-acceptors.
That H-bonds increase NLO (SHG) property can be an incorrect statement because H-bonds are present in all solids (both centrosymmetric and non-centrosymmetric) which contain H-donors and H-acceptors.
- Badges
- Science topic
- Similar topics
- Chemistry
- Organic Chemistry