Hi, Colin,
... I've been carrying a (very beaten up) 5'' x 3'' (April 1986) edition of the
LBL ‘X-ray Data Booklet’ around in my briefcase for the past ¼ century or more.
(An invaluable reference work.) So, I’m most pleased to follow the link that
you refer to now, to the online version. What progress. Fantastic !!
Definitely, though, I’ll not forsake this here little book.
Many Thanks and Very Best Regards,
Marcus.
-----Original Message-----
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Colin Nave
Sent: Tuesday, February 03, 2015 3:03 PM
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] proton scattering by X-rays
Hi Tim
Although my SHELX comment was in jest, your point illustrates the programs
versatility. You are also right about the flat(ish) form factor for the proton.
To get to a resolution where there is a cross over would require a very short
wavelength. Other processes would then dominate. A nice source for this is the
x-ray data booklet from LBL, in particular the chapter on scattering of x-rays
from electrons and atoms.
http://xdb.lbl.gov/Section3/Sec_3-1.html
Interestingly fig 3-1 in this does not include coherent scattering from nuclei
presumably because it is negligible compared with the other processes - in
practice Ian was correct in saying that a proton is effectively invisible to
x-rays of the energy we usually use.
Colin
-----Original Message-----
From: Tim Gruene [mailto:t...@shelx.uni-ac.gwdg.de]
Sent: 02 February 2015 22:08
To: Nave, Colin (DLSLtd,RAL,LSCI); ccp4bb
Subject: Re: [ccp4bb] proton scattering by X-rays
Hi Colin,
you can add f' for every atom type in SHELXL yourself, so in that sense, it has
been incorporated in SHELX. Bear in mind that the nucleus is point-like to
X-rays at ordinary wavelengths so that it should not have a form factor like
the electron cloud but a constant scattering length - just as they do for
neutron scattering.
You can do the maths at what resolution the form factor and the constant
1:1860 scattering length contribution cross. It is not ridiculously small but
nowhere near 0.8A. Charge density people may need to take this into account,
but I don't know if they do.
Cheers,
Tim
On 02/02/2015 04:03 PM, Colin Nave wrote:
> “As you say the proton itself is invisible to X-rays.”
> Not quite! The ratio of scattering between electrons and protons should go as
> the inverse square of the masses.
> Ratio of mass 1:1860, ratio of scattering 1:3459600. A small correction but
> doubtless it has been incorporated in to SHELX.
> Colin
>
>
> From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of
> Ian Tickle
> Sent: 02 February 2015 13:35
> To: ccp4bb
> Subject: Re: [ccp4bb] proton scattering by X-rays
>
>
> Peter, if it's a covalently-bonded H atom it surely can't be a bare proton,
> it must have at least some partial electron around it for the (possibly
> partial) covalent bond, enough to diffract X-rays anyway. As you say the
> proton itself is invisible to X-rays.
> Cheers
> -- Ian
>
> On 2 February 2015 at 13:08, Peter Moody
> <pcem1bigfi...@gmail.com<mailto:pcem1bigfi...@gmail.com<mailto:pcem1bigfi...@gmail.com%3cmailto:pcem1bigfi...@gmail.com>>>
> wrote:
> Dear BB
>
> I have (again) realised how limited by understanding of our subject is.
>
> In Nature’s online site
> http://www.nature.com/nature/journal/vaop/ncurrent/full/nature14110.html?WT.ec_id=NATURE-20150129
> there is a paper describing an X-ray structure determined with sub-atomic
> data (nice!). The figures show density for H+ as well as H-. In my simple
> way I had assumed that any X-ray scattering from the nucleus was negligible,
> and that the electrons are responsible for this. I would expect a proton
> (i.e. H+) alone to be invisible to X-rays, and certainly not to look similar
> to a hydride (with two electrons in (electron density) maps. What have I
> missed? Could someone please explain, or point me to a suitable reference?
>
> Best wishes, Peter
> (please use
> peter.mo...@le.ac.uk<mailto:peter.mo...@le.ac.uk<mailto:peter.mo...@le.ac.uk%3cmailto:peter.mo...@le.ac.uk>>
> to reply
> directly)
>
> http://www2.le.ac.uk/departments/biochemistry/staff/moody
>
>
>
--
Dr Tim Gruene
Institut fuer anorganische Chemie
Tammannstr. 4
D-37077 Goettingen
GPG Key ID = A46BEE1A
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