[HN Gopher] Benzene's bond lengths corrected: Previously out by ...
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Benzene's bond lengths corrected: Previously out by several
milliangstroms
Author : respinal
Score : 45 points
Date : 2022-09-26 05:05 UTC (1 days ago)
(HTM) web link (www.chemistryworld.com)
(TXT) w3m dump (www.chemistryworld.com)
| wjnc wrote:
| 1 Angstrom = 1e-10 m
|
| 11.5 mA = 1.15e-12 m
|
| That's a mind boggling measurement.
| SilasX wrote:
| So ... I'm guessing chemistry wasn't really sensitive to the
| difference, except in whatever few "edge cases" prompted this re-
| examination?
| nayuki wrote:
| > milliangstroms
|
| There is a perfectly sensible nearby SI unit called the picometre
| (pm). 10 mA = 1 pm. https://en.wikipedia.org/wiki/Picometre
| gilleain wrote:
| Yes, while the angstrom is often used for bond lengths, I've
| only heard of picometers used for smaller lengths.
| marcosdumay wrote:
| When every measure around is on the same unity, you use that as
| your base unity.
|
| If you are designing a large steel piece for milling, you will
| write a length of 1000mm, not 1m.
| kennywinker wrote:
| 1m and 1000mm are not the same length.
|
| 1000mm and 1.000m are the same length.
| marcosdumay wrote:
| Well, you wouldn't write 1.000m on it either.
|
| That one would certainly lead to you getting something
| other than you are trying to design. Unless the machinist
| misreads it, or it gets assigned to that one machinist that
| likes searching people out and talking to them.
| gog-ma-gog wrote:
| the only thing significant figures seem to be optimizing
| for is debate about the right way to use significant
| figures
|
| in my experience, it's always better to eschew them
| entirely and simply write an explicit error margin
| thehappypm wrote:
| I don't think this is correct.
|
| 1000.0 mm would be the same as 1.000m (or is it 1.0000?).
|
| 1000m has ambiguous sig figs.
| brokensegue wrote:
| Not according to
| https://en.m.wikipedia.org/wiki/Significant_figures
| foxyv wrote:
| Pretty close. If they added an additional zero to make
| 1.0000 meters they would be the same number of
| significant figures technically. But it's all convention.
| I've been in classes where 1.000 would be 4 significant
| figures.
|
| > 120.0000 consists of six significant figures (1, 2, and
| the four subsequent zeroes) except for the last zero If
| the resolution is to 0.001.
| tomjakubowski wrote:
| 1.000 has 4 significant figures. Absent other context,
| 1000 has 1.
| foxyv wrote:
| When arguing convention, everyone is right, and everyone
| is wrong.
| kennywinker wrote:
| You're right that I'm remembering my sig-fig rules wrong,
| but you're not entirely right:
|
| > Zeros to the right of the last non-zero digit (trailing
| zeros) in a number with the decimal point are significant
| if they are within the measurement or reporting
| resolution.
|
| > 1.200 has four significant figures (1, 2, 0, and 0) if
| they are allowed by the measurement resolution.
|
| and
|
| > Trailing zeros in an integer may or may not be
| significant, depending on the measurement or reporting
| resolution.
|
| So my first example "1m and 1000mm" actually _might_ be
| the same length, depending on if those zeros are
| significant or not which has not been clearly
| communicated.
|
| My second example "1000mm and 1.000m" is also ambiguous,
| since 1.000m is clear(ish) about its sig-figs, while
| 1000mm is not. I suppose I could have written "1000. mm"
|
| Best solution imo would probably be:
|
| - something that actually specified the error: 1m +-
| 0.0001m
|
| - or using words to indicate what's meant: "exactly
| 1000mm"
| tomjakubowski wrote:
| The classic solution to this ambiguity is to always use
| scientific notation.
|
| The sig figs in 1.0E3mm, 1E3mm, or 1.000000E3mm are
| unambiguous.
| swamp40 wrote:
| Same as "mils" for thousands of an inch. It's actually wildly
| popular in the US.
| nayuki wrote:
| Mils are a terrible compromise. On the one hand, they
| acknowledge that dividing by 1000 is useful and that it's
| easier to work on a linear scale instead of mixed units and
| fractions. On the other hand, it stubbornly clings onto the
| inch instead of embracing the millimetre or micrometre.
|
| As a side note, I heard of carpenters who work in decimal
| feet. Yeah, that's great, adding yet another option to the
| mix. Everyone else talks in feet and inches.
| fritzo wrote:
| ok but 'milliangstroms' immediately conveys a sense of relative
| error
| [deleted]
| jl6 wrote:
| Makes me wonder if this will result in any computational
| chemistry projects needing to be reworked.
| mch17 wrote:
| I don't think it's likely. A well implemented ab initio
| calculation will include geometry optimization.
|
| It's possible that some forcefield constants will need to be
| tweaked in MD jobs, but this is a really small change in the
| bond length.
| ta988 wrote:
| This is the paper behind that story:
| https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9347355/
| suifbwish wrote:
| I'm curious will this new knowledge will lead to new discoveries
| which might not have been possible due to working with the
| incorrect lengths in calculations?
| quantum_magpie wrote:
| As a quantum chemist, no.
| whatshisface wrote:
| It is an example of a new technique being used on a common
| chemical with an eye towards using it on other chemicals once
| the technique is proven.
|
| 'And as this technique can be applied to highly symmetric
| molecules, it opens the door to the characterisation of many
| chemically important species like polycyclic aromatic
| hydrocarbons.' - a researcher quoted in the article
| jchanimal wrote:
| My chemistry professor told me 10 Angstrom is about how long your
| fingernails grow in the time it takes to say "10 Angstrom"
| jsharf wrote:
| I just did the math, and it looks like this is correct!
|
| Assuming 2.5mm per month (quick google search), that's 2.5e+7
| angstrom/month
|
| Divide by the number of seconds in a month (30 * 24 * 3600) and
| you get about 10 angstroms per second. It takes about 1 second
| to say 10 angstroms. Very cool!
| hilbert42 wrote:
| Now that's just the type of knowledge my brain loves to
| collect and store in its useless information department (and
| it's pretty full already). Useless? Well perhaps not. But one
| thing's certain I'll never forget the fact.
|
| Incidentally, it's worth remembering we owe the beginnings of
| this story to August Kekule's remarkable insight-benzene's
| structure and the snake swallowing its tail. That too is also
| unforgettable information (but likely more useful, methinks).
| gus_massa wrote:
| Note that the length of the C-C bound is ~1.3A and the length of
| the C-H is ~1.0A, so te corrections is like .5%.
| ta988 wrote:
| The paper is really about the difference between C-H and C-D.
| Also it uses a physical method (Raman) and statistical
| approaches that will need to be reproduced to make sure it was
| not just an artifact of the method.
| fatuna wrote:
| Fun fact, even though most of the world pronounces it "angstrum",
| "Angstrom" should actually be pronounced "ongstrum" (as the
| Swedish still do).
| curiousllama wrote:
| Knowing very little about chemistry and physics, I always enjoy
| learning the names of new measurements.
|
| Milliangstroms? Lmao great name.
| nayuki wrote:
| Yes, it's a great name like millimetres of mercury (mmHg),
| kilotons of TNT, light-years, etc. /s
|
| But seriously, SI already has appropriate units for these
| quantities. Milliangstroms should be converted to picometres,
| millimetres of mercury should be converted to kilopascals,
| kilotons of TNT should be converted to terajoules, and light-
| years should be converted to petametres.
| dotnet00 wrote:
| This is a bad argument. Units exist to make things easier to
| communicate, using lightyears or kilotons makes perfect sense
| for contexts where they convey more information directly than
| petameters or terajoules. Especially since the conversions
| aren't exactly difficult to remember for anyone who needs
| them.
|
| Here the use of milliangstroms easily conveys the information
| that bond lengths are measured in angstroms and thus this is
| a very small correction, picometers would not convey that to
| anyone not already familiar with typical bond lengths.
| nayuki wrote:
| Turn this around: How many people have seen the speed of
| light, even for a second? How many people have seen a
| kiloton of TNT being blasted? You're only advocating for
| these units because they are commonly reported in the media
| and are thus familiar - at least in print, not in physical
| experience.
|
| Angstroms are needless jargon because nanometres already
| cover roughly the same scale. Do you want to go back to the
| bad old days when every town had its own definition of feet
| and pounds, and consumers were routinely cheated through
| the use of confusing and obfuscated units?
| dotnet00 wrote:
| People haven't seen the speed of light, but they have an
| understanding of the fact that it's the maximum possible
| speed. Similarly, while people haven't seen a kiloton of
| TNT being blasted, they can easily visualize what a
| kiloton of TNT is and understand what that means.
|
| Your claim that I'm only advocating these units because
| they're common in media is ignoring the fact that these
| are also very common as formal units in their respective
| fields. Another great example is AU, no one has seen what
| 1AU exactly is, but everyone can intuitively understand
| that 5AU means 5x further from the Sun than the Earth is,
| 0.74 terameters conveys absolutely nothing on its own and
| even for someone who is aware that 1AU=0.15 terameters,
| it takes a bit of extra mental effort to obtain the same
| information while not really adding anything because
| '1AU=0.15 terameters' is fundamental knowledge in the
| field anyway.
|
| This is not going back to the old days of units for two
| big reasons, they have important meaning within their
| field and there is widespread knowledge of conversion
| factors. Normalized units are also very popular in
| physics as they simplify the math and are more intuitive
| due to being directly tied to physical properties.
|
| Hell, even going back to the old metric vs imperial
| debate, there were very good benefits to the imperial
| system back when calculators were rare as multiples of
| 12s were more convenient to divide in the most common
| fractions (2, 3, 4, 6). Even now our clocks are based on
| those multiples due to their convenience for general use.
| dylan604 wrote:
| >kilotons of TNT should be converted to terajoules
|
| I disagree with this one. A ton of TNT is much more easily
| understood vs some measurement of energy that no normal human
| has any capacity of understanding.
|
| Also, milliangstroms vs picometers. if you are already using
| angstroms, then dividing it by milli makes sense.
| aaronax wrote:
| I doubt a typical person can describe the effect of one ton
| of TNT with any accuracy. I'll give it a try though: 2
| meter deep and 6 meter wide crater in typical soil, death
| due to shockwave out to 150m, death likely due to shrapnel
| out to 300m, buildings destroyed out to 300m, audible from
| 10 km.
|
| From here: https://unsaferguard.org/un-saferguard/blast-
| damage-estimati...
|
| 26 meters fatal distance, 83 or 142 meters buildings
| destroyed
|
| My estimates are more appropriate for a 20-30 ton
| explosion, it seems.
| dylan604 wrote:
| You think the typical person can tell the effect of
| whatever value in terajoules would be? At least the
| typical person would know that dynamite/TNT is an
| explosive causing damage and that can be extrapolated in
| one's mind the effect of 1000 tons of TNT would be much
| larger and so forth.
| namanyayg wrote:
| How is 1 light year = 1 petameter?
| drekk wrote:
| They're not. We're saying the non-SI unit of light years
| shouldn't be used over something like petameter. Just
| multiply the "light year" measurement by .9461 and you're
| set. Or, ideally, just measure in SI units to begin with?
| The US is the holdout here and for no good reason
| dylan604 wrote:
| How long does it take for light to travel petameter vs
| how long did it take to travel that light year? The light
| year is something people can relate to
| 12ian34 wrote:
| I for one welcome the idea of measuring time in
| lightpetametres ;)
|
| (that's about 38.6 days for the curious)
| Maursault wrote:
| The problem is with what a year is, which is arbitrary to
| begin with, but worse still, _a year is not constant_.
| The momentum of every planet is affected by the gravity
| of every other planet, leading to fluctuations in solar
| orbital speed, which means a full orbit is likely never
| the same period as the previous or subsequent orbit.
| Maybe these fluctuations are small, but light travels so
| fast that over long distances it is sure to be
| significant. Thus the definition of a light year is
| defined as the distance light travels in a Julian year,
| 365.25 days, but this just pushes the error down to a
| different measure, because days are not constant either.
| Since the speed of light actually is constant, maybe we
| should define years and days by the speed of light
| instead of vice versa.
| dylan604 wrote:
| Of course, there's also the problem that a light year is
| only relevant to someone from Earth. A light year for a
| Neptunian would be drastically longer, let alone someone
| extra-solar system to us
| beojan wrote:
| The US isn't the holdout. Astronomers everywhere use
| light years and parsecs. Particle physicists everywhere
| use barns. Chemists everywhere use Angstroms.
| dylan604 wrote:
| Astronomers also use angstroms when refering to the
| filters used for solar observations.
|
| Just because they're pretty, here's a link to the SDO
| gallery: https://sdo.gsfc.nasa.gov/gallery/main
|
| Also, the SDO takes an image through each filter as it
| rotates through the filter wheel, IIRC, it takes 15
| minutes to rotate through them all. They are then made
| available for each filter at different resolutions. Being
| the hacker-ish type, I wrote a script to use curl to
| download the image sequences for chosen filter/framesize
| for the sun's full rotation to create a one day
| timelapse.
| idiotsecant wrote:
| 'everyone should use the units I want them to use and not the
| commonly used ones'
| hypertele-Xii wrote:
| That's how we get standards.
| nayuki wrote:
| This is a disingenuous argument. Do you not see the
| problems with using application-specific units? They are
| jargon and don't form a coherent system.
|
| When you have a jumble of units, it is hard to relate them.
| If an acre of land gets an inch of rain, how many gallons
| of water is that? If you did this in millimetres, metres,
| and cubic metres, it is easy.
|
| If your car's motor is rated in horsepower but electricity
| is measured in kilowatts, how do you compare those?
|
| What about fitting copies of a product measured in ounces
| in a shipping container measured in short tons?
|
| Typesetting measured in points, but paper measured in
| inches?
|
| Fitting metalwork done in mils but woodwork done in binary-
| fraction inches in a house measured in feet?
|
| The list goes on and on. It's easy to say "but my domain is
| special and deserves its own units". It's much harder to
| resist and say no, there is a perfectly suitable unit with
| power-of-1000 prefixes to use in your application.
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